A practical guide to Fedora and Red Hat Enterprise Linux, 7th Edition (2014)

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Objectives

After reading this chapter you should be able to:

Discuss a variety of network types

Define several network protocols

List the software and hardware components of networks

Explain the features and advantages of IPv6

List several network utilities

Explain how ping works

Use dig to determine the nameserver for a Web site

Describe distributed computing

Explain the role of the World Wide Web

Introduction to Networking

The communications facilities linking computers are continually improving, allowing faster and more economical connections. The earliest computers were unconnected stand-alone systems. To transfer information from one system to another, you had to store it in some form (usually magnetic tape, paper tape, or punch cards—called IBM or Hollerith cards), carry it to a compatible system, and read it back in. A notable advance occurred when computers began to exchange data over serial lines, although the transfer rate was slow (hundreds of bits per second). People quickly invented new ways to take advantage of this computing power, such as email, news retrieval, and bulletin board services. With the speed and ubiquity of today's networks, a piece of email can cross the country or even travel halfway around the world in a fraction of a second.

Today it would be difficult to find a computer facility that does not include a LAN to link its systems. Linux systems are typically attached to an Ethernet (page 1249) network. Wireless networks are also prevalent. Large computer facilities usually maintain several networks, often of different types, and almost certainly have connections to larger networks (companywide or campuswide and beyond).

Internet

The Internet is a loosely administered network of networks (an internetwork) that links computers on diverse LANs around the globe. An internet (small i) is a generic network of networks that might share some parts in common with the public Internet. It is the Internet that makes it possible to send an email message to a colleague thousands of miles away and receive a reply within minutes. A related term, intranet, refers to the networking infrastructure within a company or other institution. Intranets are usually private; access to them from external networks might be limited and carefully controlled, typically using firewalls (page 288).

Network services

Over the past decade many network services have emerged and become standardized. On Linux and UNIX systems, special processes called daemons (page 1245) support such services by exchanging specialized messages with other systems over the network. Several software systems have been created to allow computers to share filesystems with one another, making it appear as though remote files are stored on local disks. Sharing remote filesystems allows users to share information without knowing where the files physically reside, without making unnecessary copies, and without learning a new set of utilities to manipulate them. Because the files appear to be stored locally, you can use standard utilities (e.g., cat, vim, lpr, mv, or their graphical counterparts) to work with them.

Many tools take advantage of networks for more than sharing files. These tools have varied tasks, from executing programs remotely to communicating between computers, programs, and people. The ssh (secure shell, page 685) utility is the standard tool for remote command execution and remote computer access. Years ago it replaced insecure tools such as rsh, telnet, and rlogin. The ssh utility encompasses the features of these tools and includes builtin security features for safe and private access across public networks. Some devices do not implement ssh, but not many.

Previously, users communicated using command-line tools such as talk, write, and IRC (Internet Relay Chat). These tools are still around, particularly IRC, but their use has waned in preference to Facebook, Twitter, Google Talk, Skype, and older IM (Instant Messaging) tools like MSN, AIM, ICQ, and Yahoo Messenger. Many people use the Empathy IM client for Linux. Jabber is popular in private company networks.

Intranet

An intranet is a network that connects computing resources at a school, company, or other organization but, unlike the Internet, typically restricts access to internal users. An intranet is usually composed of one or more local area networks (LANs) but could be fairly large for a regional, national, or worldwide company. An intranet can provide database, email, and Web page access to a limited group of people, regardless of their geographic location.

The ability of an intranet to connect dissimilar machines is one of its strengths. Think of all the machines you can find on the Internet: Macintosh systems, PCs running different versions of Windows, machines running UNIX and Linux, and so on. Each of these machines can communicate via the IP protocol suite (page 290). The intranet defines the communication boundaries and security trust zones that enable this communication.

Another key difference between the Internet and an intranet is that the Internet transmits only one protocol suite natively: IP. In contrast, an intranet can be set up to use a number of protocols, such as IP, AppleTalk, or other protocols developed by vendors over the years. Although these protocols cannot be transmitted directly over the Internet, you can set up gateway boxes at remote sites that tunnel or encapsulate these protocols into IP packets and then use the Internet to pass them. In practice, most of the older protocols have been rewritten to use IP as a transport. Even storage protocols and networks like SCSI and Fibre Channel have IP implementations now (iSCSI, FCoE).

You can use an extranet (also called a partner net) or a VPN (virtual private network) to improve security. These terms describe ways to connect remote sites securely to a local site, typically by using the public Internet as a carrier and employing encryption as a means of protecting data in transit. A typical use of an extranet is to link research institutions for collaboration or to link a parts supplier and a manufacturer to obtain direct inventory access.

Following are some terms you might want to become familiar with before you read the rest of this chapter:

ASP (page 1237)

bridge (page 1240)

extranet (page 1249)

firewall (page 1250)

gateway (page 1251)

hub (page 1254)

internet (page 1255)

Internet (page 1255)

intranet (page 1255)

ISP (page 1256)

packet (page 1265)

router (page 1271)

sneakernet (page 1273)

switch (page 1276)

VPN (page 1280)

Types of Networks and How They Work

Computers communicate over IP networks using unique addresses assigned by system software. An IP packet includes the address of the destination computer and the sender’s return address. Networks can consist of many distinct topologies, or arrangements of computers and networking equipment. The most common topologies are broadcast, point-to-point, and switched. Ethernet is the most common topology used in LANs (local area networks, discussed shortly). Ethernet speeds range from 10 megabits per second to 100 gigabits per second (not formally accepted as of this writing). Other types of LANs include Myrinet, Infiniband, and Quadrics, but they are used mostly in high-performance computing.

Speed is critical to the proper functioning of the Internet. Newer specifications (cat 6 and cat 7) are being adopted for 10000BaseT (10 gigabits per second; also called 10GE) and faster networking. Specialized cables are required for higher speeds. SFP+ and QSFP are in common use. Many of the networks that form the backbone of the Internet run at speeds of 38 gigabits per second (OC768) to accommodate the ever-increasing demand for network services. Table 8-1 lists some of the specifications in use today.

Table 8-1 Network specifications

Broadcast Networks

On a broadcast network, any of the many systems attached to the network cable can send a message at any time; each system examines the destination address in each message and responds only to messages addressed to it. A problem occurs on a broadcast network when multiple systems send data at the same time, resulting in a collision of the messages on the cable. When messages collide, they can become garbled. The sending system notices the garbled message and resends it after waiting a short but random amount of time. Waiting a random amount of time helps prevent those same systems from resending the data at the same moment and experiencing yet another collision. The extra traffic that results from collisions can strain the network; if the collision rate gets too high, retransmissions might result in more collisions. Ultimately the network might become unusable.

A special broadcast (page 1240) packet in a network requires all machines to inspect the packet. Certain protocols take advantage of this feature: The ARP protocol, which is used for IP addresses to MAC (Ethernet) addresses resolution, the PXE network boot protocol, DHCP, and some service discovery protocols all use broadcast packets.

Point-to-Point Networks

A point-to-point link does not seem like much of a network because only two endpoints are involved. However, most connections to WANs (wide area networks) go through point-to-point links, using wire cable, radio, or satellite links. The advantage of a point-to-point link is its simplicity: Because only two systems are involved, the traffic on the link is limited and well understood. A disadvantage is that each system can typically be equipped for only a small number of such links; it is impractical and costly to establish point-to-point links that connect each computer to all the rest. For example two hosts require one link, three hosts require three links, four hosts require six links, and five hosts require ten links to connect a point-to-point network. The number of required links increases more quickly as you add more hosts. Point-to-point links often use serial links and specialized interfaces.

The most common types of point-to-point links are the ones used to connect to the Internet. When you use DSL¹ (digital subscriber line), you are using a point-to-point link to connect to the Internet. Serial lines, such as T-1, T-3, ATM links, FIOS, and ISDN, are all point-to-point. Although it might seem like a point-to-point link, a cable modem is based on broadcast technology and in that way is similar to Ethernet.

1. The term DSL incorporates the xDSL suite of technologies, which includes ADSL, IDSL, HDSL, SDSL, and XDSL.

Switched Networks

With the introduction of switched networks, pure broadcast networks became a thing of the past. A switched network runs in full duplex mode, allowing machines and network equipment to send and receive data at the same time. Support for full duplex is built-in and required in gigabit networks and above, on 100-megabit networks it is optional, and on 10-megabit networks it is rare.

A switch is a device that establishes a virtual path between source and destination hosts in such a way that each path appears to be a point-to-point link, much like an old-fashioned telephone switchboard. Instead of the operator using a plug to connect your local line to an outgoing line, the switch inspects each packet to determine its destination and directs the packet appropriately.

The switch creates and tears down virtual paths as hosts seek to communicate with each other. Each host thinks it has a direct point-to-point path to the host it is talking to. Contrast this approach with a broadcast network, where each host also sees traffic bound for other hosts. The advantage of a switched network over a pure point-to-point network is that each host requires only one connection: the connection to the switch. Using pure point-to-point connections, each host must have a connection to every other host. Scalability is provided by further linking switches.

To achieve this reduction in traffic, Ethernet switches learn which hosts are on which ports. When traffic comes in destined for a host, the switch looks up which port that host is connected to in its CAM² and sends the packet out that port. When a switch has not yet heard from a host matching the destination, or if the packet is a special broadcast packet, the packet is flooded out on all ports just like a packet in a broadcast network. A switched Ethernet network is a technological enhancement over a broadcast network.

2. A memory table used to dereference (page 1246) Ethernet addresses.

LAN: Local Area Network

Local area networks (LANs) are confined to a relatively small area—a single computer facility, building, or campus. Today most LANs run over copper. Fiberoptic (glass or plastic) cable, wireless (Wi-Fi), and sometimes infrared (similar to most television remote control devices) are also common infrastructure.

If its destination address is not on the local network, a packet must be passed on to another network by a router (page 287). A router might be a general-purpose computer or a special-purpose device attached to multiple networks to act as a gateway among them.

Switching terminology

You might see references to a layer-2 switch, layer-3 switch, or layer-4 switch. These terms refer to the IP networking model (page 290). A layer-2 switch is what the preceding discussion of switched networks describes. A layer-3 switch is equivalent to a router; it combines features of layer-2 switching and routing. A single layer-3 switch can connect multiple independent LANs. Layer-4 switches inspect higher-level packets to make decisions. Most Web load balancers are layer-4 switches. They can route traffic to many devices to spread the load on the network.

Ethernet

A Linux system connected to a LAN usually connects to a network using Ethernet. A typical Ethernet connection can support data transfer rates from 10 megabits per second to 100 gigabits per second. Transfer rates of 10 megabits per second, 100 megabits per second, and 1 gigabit per second use the same, older technology. However, 10, 40, and 100 gigabit per second transfer rates require newer technology and specialized cables. Ten gigabit per second Ethernet can be compatible with 1 gigabit per second Ethernet by using physical media adapters. Because of the need for extremely tight tolerances, 40 and 100 gigabit per second Ethernet are not backward compatible. The technology required for these higher-speed transfer rates requires very precisely defined physical interfaces. The hardware is quite expensive and common only in the network infrastructure realm and HPC (high performance computing) environments, connecting multiple switches using high-speed links.

Cables

As mentioned earlier, a modern Ethernet network transfers data using copper or fiberoptic cable or wireless transmitters and receivers. Originally, each computer was attached to a thick coaxial cable (called thicknet) at tap points spaced at six-foot intervals along the cable. The thick cable was awkward to deal with, so other solutions, including a thinner coaxial cable called thinnet, or 10Base2,³ were developed.

3. Versions of Ethernet are classified as XBaseY, where X is the data rate in megabits per second, Base means baseband (as opposed to radio frequency), and Y is the category of cabling.

Today most Ethernet connections are either wireless or made over UTP (unshielded twisted pair). There are a number of UTP standards, each referred to by a category and a number. Some examples are Category 5 [cat 5], Category 5e [cat 5e], and Category 6 [cat 6]. These categories specify how far the cable can carry a signal. The higher the number, the tighter the tolerances and the more expensive the cable, but the farther it can reliably carry a signal. These standards specify the physical connectors at the ends of the cables, how tightly the wires in the cable are twisted, and various other parameters. The terms 10BaseT, 100BaseT, and 1000BaseT refer to Ethernet over cat-3/4/5/5e/6/7 cables. STP (shielded twisted pair) is not very common.

Segment

A network segment is a part of a network in which all systems communicate using the same physical layer (layer 1) of the IP and OSI models (page 291). It is of arbitrary size and can be a part of a WAN, MAN, or another network.

Duplex

In half-duplex mode, packets travel in one direction at a time over the cable. In full-duplex mode, packets travel in both directions.

Hub

A hub (sometimes called a concentrator) is a device that connects systems so they are all part of one network segment and share the network bandwidth. Hubs work at the physical layer of the IP and OSI models (layer 1, page 291). All packets are sent to all hosts (flooded). For the most part, hubs have been replaced by switches.

Switch

A switch connects network segments. A switch inspects each data packet; it learns which devices are connected to which of its ports. The switch sends each packet only to the device it is intended for. Because a switch sends packets only to their destination devices, it can conserve network bandwidth and perform better than a hub. Some switches have buffers that hold and queue packets.

Switches work at layers 2 and higher of the IP and OSI models (page 290). A layer-2 switch works at the data link layer, and a layer-3 switch works at the IP layer and routes packets. Layer-4 switches work at the transport and application layers and are the basis for load balancers and application proxies.

All modern Ethernet switches have enough bandwidth to communicate simultaneously, in full-duplex mode, with all connected devices. A nonswitched (hub-based) broadcast network can run in only half-duplex mode. Full-duplex Ethernet further improves efficiency by eliminating collisions. Each host on a full-duplex switched network can transmit and receive simultaneously at the speed of the network (e.g., 100 megabits per second) for an effective bandwidth between hosts of twice the speed of the network (e.g., 200 megabits per second), depending on the capacity of the switch.

Bridge

A network bridge connects multiple network segments at the data link layer (IP layer 2) of the OSI model. A bridge is similar to a repeater or network hub, devices that connect network segments at the physical layer; however, a bridge works by forwarding traffic from one network segment to another only if the destination device specified in the packet is known to be on the remote segment. A bridge does not forward traffic between LAN hosts on the same side of the bridge.

In Ethernet networks, the term bridge formally means a device that behaves according to the IEEE 802.1D standard. Marketing literature frequently refers to this type of device as a network switch.

Router

A router connects networks at layer 3 of the IP and OSI models (page 291). For example, a router can connect a LAN to a WAN (such as the Internet) or it can connect two LANs. A router determines which path packets should take to travel to a different network and forwards the packets. Routers work at the network layer of the IP and OSI models (layer 3). “Internetworking Through Gateways and Routers” on the next page covers routers in more depth.

VLAN

A VLAN (virtual local area network or virtual LAN) is a logical entity defined in software. It defines a group of hosts that share the same broadcast domain (layer 2). That is, when a host sends out a broadcast packet, such as an ARP packet, it will arrive at all other hosts in the same VLAN. All modern, managed switches, those that have a command line or Web interface, have VLAN capability. A VLAN allows an administrator to group hosts into IP ranges (LANs). VLANs commonly group hosts by department, organization, or security needs. Hosts in the same VLAN communicate with each other using layer 2, and hosts in different VLANs communicate using layer 3. The primary difference between a LAN and a VLAN is that in a LAN (a nonmanaged switch), all hosts are in the same broadcast domain, whereas in a VLAN, there can be multiple broadcast domains on a single switch.

Broadcast domains and IP protocol layer 2 segments are effectively the same thing. However, layer 2 can be confused with hosts (network addresses) being in the same IP network. Consider the classic concentrator, a device that sends out all packets on all ports all the time. These ports might include separate networks. For example, some hosts might be in network 10.0.1.0, and others might be in network 192.168.1.0. Ethernet broadcast packets do not know the difference between the networks; these packets go to all hosts regardless of IP address. This setup is sometimes referred to as a collision domain. VLANs enable you to place the hosts in the IP network 10.0.1.0 into one VLAN and the hosts in 192.168.1.0 into another, so the networks do not share each other’s broadcasts. VLANs also enable you to connect hosts that share the same function across a corporate network.

Wireless

Wireless networks are becoming increasingly common. They are found in offices, homes, and public places, such as universities, coffee shops, and airports. Wireless access points provide functionality similar to an Ethernet hub. They allow multiple users to interact via a common radio frequency spectrum. A wireless, point-to-point connection allows you to wander about your home or office with a laptop, using an antenna to link to a LAN or to the Internet via an in-house base station. Linux includes drivers for many wireless devices. A wireless access point, or base station, connects a wireless network to a wired network so that no special protocol is required for a wireless connection. Refer to the Linux Wireless LAN HOWTO at www.hpl.hp.com/personal/Jean_Tourrilhes/Linux.

WAN: Wide Area Network

A WAN (wide area network) covers a large geographic area. In contrast, the technologies (such as Ethernet) used for LANs were designed to work over limited distances and for a certain number of host connections. A WAN might span long distances over dedicated data lines (leased from a telephone company) or radio or satellite links. Such networks are often used to connect LANs and typically support much lower bandwidth than LANs because of the expense of the connection. Major Internet service providers rely on WANs to connect to their customers within a country and around the globe.

MAN

Some networks do not fit into either the LAN or the WAN designation. A metropolitan area network (MAN) is a network that is contained in a smaller geographic area, such as a city. Like WANs, MANs are typically used to interconnect LANs.

Internetworking Through Gateways and Routers

Gateway

A LAN connects to a WAN through a gateway, a generic term for a computer or a special device with multiple network connections that passes data from one network to another. A gateway connects a LAN to other LANs, VLANs, or to a WAN. Data that crosses the country from one Ethernet to another over a WAN, for example, is repackaged from the Ethernet format to a different format that can be processed by the communications equipment that makes up the WAN backbone. When it reaches the end of its journey over the WAN, the data is converted by another gateway to a format appropriate for the receiving network. For the most part, these details are of concern only to the network administrators; the end user does not need to know anything about how the data transfer takes place.

Router

The modern, canonical reference to a gateway is to the default gateway, which is the router that connects a LAN or VLAN to other networks. Routers play an important role in internetworking. Just as you might study a map to plan your route when you need to drive to an unfamiliar place, so a computer needs to know how to deliver a message to a system attached to a distant network by passing through intermediary systems and networks along the way. Although you might envision using a giant network road map to choose the route that your data should follow, a static map of computer routes is usually a poor choice for a large network. Computers and networks along the route you choose might be overloaded or down and not provide a detour for your message.

Routers instead communicate dynamically, keeping each other informed about which routes are open for use. To extend the analogy, this situation is like heading out on a car trip without consulting a map to find a route to your destination; instead you head for a nearby gas station and ask directions. Throughout the journey you continue to stop at one gas station after another, getting directions at each to find the next one. Although it would take a while to make the stops, the owner of each gas station would advise you of bad traffic, closed roads, alternative routes, and shortcuts.

The stops made by the data are much quicker than those you would make in your car, but each message leaves each router on a path chosen based on the most current information. Think of this system as a GPS (global positioning system) setup that automatically gets updates at each intersection and tells you where to go next, based on traffic and highway conditions.

Figure 8-1 shows an example of how LANs might be set up at three sites interconnected by a WAN (the Internet). In this type of network diagram, Ethernet LANs are drawn as straight lines, with devices attached at right angles; WANs are represented as clouds, indicating the details have been left out; and wireless connections are drawn as zigzag lines with breaks, indicating the connection might be intermittent.

Figure 8-1 A slice of the Internet

In Figure 8-1, a gateway or a router relays messages between each LAN and the Internet. The figure shows the three routers in the Internet that are closest to each site. Site A has a server, a workstation, a network computer, and a PC sharing a single Ethernet LAN. Site B has an Ethernet LAN that serves a printer and four Linux workstations. A firewall permits only certain traffic to pass between the Internet router and the site’s local router. Site C has three LANs linked by a single router, perhaps to reduce the traffic load that would result if the LANs were combined or to keep workgroups or locations on separate networks. Site C also includes a wireless access point that enables wireless communication with nearby computers.

Firewall

A firewall in a car separates the engine compartment from the passenger compartment, protecting the driver and passengers from engine fires, noise, and fumes. In much the same way, computer firewalls separate computers from malicious and unwanted users.

A firewall prevents certain types of traffic from entering or leaving a network. For example, a firewall might prevent traffic from your IP address from leaving the network and prevent anyone except users from selected domains from using FTP to retrieve data from the network. The implementations of firewalls vary widely—from Linux machines with two interfaces (page 1255) running custom software to a router (preceding section) with simple access lists to esoteric, vendor-supplied firewall appliances. Most larger installations have at least one kind of firewall in place. A firewall is often accompanied by a proxy server/gateway (page 316) that provides an intermediate point between you and the host you are communicating with.

In addition to the firewalls found in multipurpose computers, firewalls are becoming increasingly common in consumer appliances. For example, they are built into cable modems, wireless gateways, routers, and stand-alone devices.

Typically a single Linux machine will include a minimal firewall. A small group of Linux systems might have an inexpensive Linux machine with two network interfaces and packet-filtering software functioning as a dedicated firewall. One of the interfaces connects to the Internet, modems, and other outside data sources. The other connects, normally through a hub or switch, to the local network. Refer to Chapter 25 for information on firewalld, firewall-config, iptables, and setting up a firewall and to Appendix C for a discussion of security.

Network Protocols

TCP

To exchange information over a network, computers must communicate using a common language, or protocol (page 1267). The protocol determines the format of message packets. The predominant network protocols used by Linux systems are TCP and IP,⁴ collectively referred to as TCP/IP (Transmission Control Protocol and Internet Protocol). Network services that require highly reliable connections, such as ssh and scp, tend to use TCP/IP.

4. All references to IP imply IPv4 (page 1256).

UDP

Network services that do not require guaranteed delivery but require timely delivery, such as video, audio, and time services, operate using the simpler UDP (User Datagram Protocol; UDP/IP). VoIP (voice over IP) and NTP (Network Time Protocol) fall into this category. UDP packets are sent and then forgotten. Voice and video protocols are delay sensitive, not integrity sensitive. The human ear and eye accept and interpolate loss in an audio or video stream but cannot deal with variable delay. The guaranteed delivery that TCP provides can introduce a delay on a busy network when packets are retransmitted. This delay is not acceptable for video and audio transmissions, whereas less than 100 percent integrity is acceptable. In the case of NTP, missing packets are acceptable, but packets that are delayed because of TCP retransmission can result in significantly skewed time settings.

IP: Internet Protocol

Layering was introduced to facilitate protocol design: Layers distinguish functional differences between adjacent protocols. A grouping of layers can be standardized into a protocol model. IP has a model that distinguishes protocol layers and that differs from the ISO seven-layer protocol model (also called the OSI model) often illustrated in networking textbooks. Specifically IP uses the following simplified five-layer model:

1. The first layer of the IP protocol, called the physical layer, describes the physical medium (e.g., copper, fiber, wireless) and the data encoding used to transmit signals on that medium (e.g., pulses of light, electrical waves, or radio waves).

2. The second layer, called the data link layer, covers media access by network devices and describes how to put data into packets, transmit the data, and check it for errors. Ethernet is found at this layer, as is 802.11 (page 1236) wireless.

3. The third layer, called the network layer, frequently uses IP and addresses and routes packets. It allows data to traverse the networks.

4. The fourth layer, called the transport layer, is where TCP and UDP exist. This layer provides a means for applications to communicate with each other. Functions commonly performed by the transport layer include guaranteed delivery, delivery of packets in the order of their transmission, flow control, error detection, and error correction. The transport layer is responsible for dividing data streams into packets. In addition, this layer performs port addressing, which allows it to distinguish among different services using the same transport protocol. Port addressing keeps the data from multiple applications using the same protocol (for example, TCP) separate.

5. Anything above the transport layer is the domain of the application and is part of the fifth layer. Unlike the ISO model, the Internet model does not distinguish among application, presentation, and session layers. All the upper-layer characteristics, such as character encoding, encryption, and GUIs, are part of the application. Applications choose the transport characteristics they require as well as the corresponding transport layer protocol with which to send and receive data.

TCP: Transmission Control Protocol

TCP is most frequently run on top of IP in a combination referred to as TCP/IP. This protocol provides error recovery and guaranteed delivery in packet transmission order; it also works with multiple ports so that it can handle more than one application. TCP is a connection-oriented protocol(page 1244), also known as a stream-based protocol. Once established, a TCP connection looks like a stream of data, not individual IP packets. The connection is assumed to remain up and be uniquely addressable. Every piece of information you write to the connection always goes to the same destination and arrives in the order it was sent. Because TCP is connection oriented and establishes a virtual circuit between two systems, this protocol is not suitable for one-to-many transmissions (see the discussion of UDP, following). TCP has builtin mechanisms for dealing with congestion (or flow) control over busy networks and throttles back (slows the speed of data flow) when it has to retransmit dropped packets. TCP can also deal with acknowledgments, wide area links, high-delay links, and other situations.

UDP: User Datagram Protocol

UDP runs at layer 4 of the IP stack, just as TCP does, but is much simpler. Like TCP, UDP works with multiple ports and multiple applications. It has checksums for error detection but does not automatically retransmit datagrams (page 1246) that fail the checksum test. UDP is a datagram-oriented protocol: Each datagram must carry its own address and port information. Each router along the way examines each datagram to determine the destination, one hop at a time. You can broadcast or multicast UDP datagrams to many destinations at the same time by using special addresses.

PPP: Point-to-Point Protocol

PPP provides serial line point-to-point connections that support IP. It compresses data to make the most of the limited bandwidth available on these connections. PPP acts as a point-to-point layer 2/3 transport that many other types of protocols can ride on. Today it is used mostly in devices such as cable modems. Previously, it was used as a transport for TCP and UDP on dial-up modems that connected a computer to the Internet.

IPv4

Under IPv4, the network address of a machine is an IP address that is represented as one number broken into four octets⁵ separated by periods (for example, 192.168.184.5). Domain names and IP addresses are assigned through a highly distributed system coordinated by ICANN (Internet Corporation for Assigned Names and Numbers—www.icann.org) via many registrars (see www.internic.net). ICANN is funded by the various domain name registries and registrars and by IP address registries, which supply globally unique identifiers for hosts and services on the Internet. Although you might not deal with any of these agencies directly, your Internet service provider most assuredly does.

5. Using binary notation, an eight-bit byte can represent the range of 0–255, thus the term octet.

How a company uses IP addresses is determined by the system or network administrator. For example, the two leftmost sets of numbers in an IP address might represent a large network (campuswide or companywide); the third set, a subnetwork (perhaps a department or a single floor in a building); and the rightmost number, an individual computer. The operating system uses the address in a different, lower-level form, converting it to its binary equivalent, a series of 1s and 0s. Refer to “Private address space” on page 637 for information about addresses you can use on a LAN without registering them.

IPv6