Network Layer, Layer-3 Addressing

Network Layer
Layer 3 of the OSI Reference Model is the network layer. This layer is responsible for three main functions:
■ Defines logical addresses used at layer-3
■ Finds paths, based on the network numbers of logical addresses, to reach destination devices
■ Connects different data link types together, such as Ethernet, FDDI, Serial, and Token Ring

The following sections cover the network layer in more depth.

Layer-3 Addressing
Many protocols function at the network layer: AppleTalk, DECnet, IP, IPX, Vines, XNS, and others. Each of these protocols has its own method of defining logical addressing. Correct assignment of these addresses on devices across your network allows you to build a hierarchical design that can scale to very large sizes. This provides an advantage over layer-2 addresses, which use a flat design and are not scalable.

All layer-3 addressing schemes have two components: network and host (or node). Each segment (physical or logical) in your network needs a unique network number. Each host on these segments needs a unique host number from within the assigned network number. The combination of the network and host number assigned
to a device provides a unique layer-3 address throughout the entire network. For example, if you had 500 devices in your network that were running IP, each of these devices would need a unique IP layer-3 address.

This process is different with MAC addresses, which are used at layer-2. MAC addresses need to be unique only on a physical (or logical) segment. In other words, within the same broadcast domain, all of the MAC addresses must be unique. However, MAC addresses do not need to be unique between two different broadcast domains.

An example of this appears later in this chapter.
To understand the components of layer-3 addresses, let’s look at a few examples. TCP/IP addresses are 32 bits in length. To make these addresses more readable, they are broken up into four bytes, or octets, where any two bytes are separated by a period. This is commonly referred to as dotted decimal notation. Here’s a simple example of an IP address: 10.1.1.1. An additional value, called a subnet mask, determines the boundary between the network and host components of an address. When comparing IP addresses to other protocols’ addressing schemes, IP is the most complicated. IP addressing is thoroughly covered in Chapter 3.

Most other protocols have a much simpler format. For example, IPX addresses are 80 bits in length. The first 32 bits are always the network number, and the last 48 bits are always the host address. IPX addresses are represented in hexadecimal. Here’s an example: ABBA.0000.0000.0001. In this example, ABBA is the network number and 0000.0000.0001 is the host number. Every protocol has its own addressing scheme. However, each scheme always begins with a network component followed by a host component.