Linux Administration (2016)

Networking

In this chapter, you will learn about the TCP/IP protocol and the most important aspects of IP networking. You'll also be introduced to network classes and classful networks. Additionally, you'll be learning about subnet masks and broadcast addresses. Next you'll learn about classless interdomain routing and, finally, you'll learn what IP addresses are meant to be used on private networks.

Today, TCP/IP is the de facto standard for transmitting data over networks. TCP/IP stands for Transmission Control Protocol (TCP) and Internet Protocol (IP). TCP is responsible for establishing and maintaining network conversations so that two devices can exchange data. The Internet Protocol is responsible for sending data from one device to another device on a network. Each one of these network devices is known as a host and has at least one IP address.

For a device on a network to communicate properly, it needs three pieces of information: an IP address, a subnet mask, and a broadcast address. Each one of these numbers is comprised of four octets separated by a dot. An octet represents eight-bits and therefore can have a value starting at 0 and going up to 255.

Example IP Address: 199.83.131.168

Example Subnet Mask: 255.255.255.0

Example Broadcast Address: 199.83.131.255

In the example IP Address of 199.83.131.186, the first octet contains a value of 199, the second octet is 83, the third octet is 131, and the last octet is 186.

IP addresses are comprised of two parts. The first part of an IP address is the network address and the second part is the host address. The network portion of the IP address tells routers what network the host belongs to and thus where to route data that is destined for that host. The host address tells routers the specific device that the data should be sent to. For routing to work properly, each group of devices, or network, needs to have a unique network address. Also, each device within that network needs to have a unique host address.

The class of an address determines what portion is used as the network address and what portion is used for host addresses.

By looking at the first octet of an IP address, you can tell what class it is. An IP address with a first octet that falls between 1 and 127 is a class A IP address. For example, the IP address 17.24.88.9 belongs to a class A network. Class B addresses begin with 128.0 and end at 191.255. For example, 183.194.46.31 belongs to a class B network. Class C addresses start with 192.0.0 and end with 233.255.255. An example IP address that belongs to a class C network is 199.83.131.186.

A class determines the possible number of networks and the addressable space per network. For example, a Class A network can accommodate about 16 million host addresses. A Class B network can have up to 65,536 hosts in it, and a class C network can address 255 hosts.

The following table lists the subnet mask used for each of the network classes. The network portion of an IP address corresponds to the 255s in the subnet mask. For example, the first octet of a Class A network is the network portion while the three remaining octets are the host portion. For Class B networks, the first two octets are for network addresses while the last two octets are for host addresses. Finally, Class C networks use the first three octets for the network and just the last octet for the host addresses.