IP as the Postmaster General of the Network

IP as the Postmaster General of the Network

The U.S. Postal Service (USPS) uses an address system that consists of a number, street name on which the building is located, town or city, state, and zip code. The USPS intended for each address in the country to be unique in some way. In a given state, all towns must have different names. In each town, each street must have a different name. Finally, on each street, each building must have a different number. This system allows each address in the country to be unique. By having unique addresses, the postal service doesn't get confused about where to send a letter.

IP is the TCP/IP protocol that most closely matches Layer 3 in the OSI networking model. IP defines addressing, as well as routing, including the underlying details of the routing example in Figure 10-1. Like the postal service, IP defines addresses so that they have structure, allowing easy routingthe networking equivalent of mail sorting. IP also states that everyone should have a unique IP address (just like the USPS requires unique postal addresses) to avoid confusion when trying to deliver data to that address. The postal service defines addressing details so that letter carriers can easily and efficiently deliver the mail; similarly, IP defines IP addressing details to facilitate easy and efficient forwarding of IP packets.

Each network interface on a computer needs an IP address. A network interface is simply a card inside a computer that has a physical connector for some type of network, such as an Ethernet network interface card (NIC). The NIC takes care of TCP/IP network interface layer details, which are the equivalent of OSI Layers 1 and 2. The TCP/IP internetworking layer, implemented by the IP protocol, defines these logical IP addresses. Most end user computers have a single network interface, meaning that the computer has only one IP address. However, any device that has at least one IP address can send and receive IP packets and is considered to be a TCP/IP host, or simply host.

IP addresses are 32-bit binary numbers, but because humans would find it inconvenient to write down 32-bit numbers, the addresses are written in decimal. The format of IP addresses is often called canonical format; sometimes it's called dotted decimal format. For example, the next two lines show the binary version of an IP address, followed by the same IP address written as a dotted-decimal number. It's obvious from comparing the two that, given a choice, it's much easier to work with the decimal version:

00001000 00000100 00000010 000000001

8.4.2.1

Each of the decimal numbers in an IP address is called an octet. (The term octet is just another term for the word byte.) So, each decimal octet represents 8 bits of the IP address, and with 4 octets (separated by periods), the whole dotted decimal IP address represents 32 bits.

Appendix B, "Converting IP Addresses Between Decimal and Binary," explains how you can convert back and forth between the binary and decimal versions of an IP address. You really don't need to think about IP addresses in binary to learn what's covered in this chapter. However, if you would like to understand more about converting between binary and decimal formats of IP addresses, feel free to read Appendix B.