Version 3 (POP3)
The TCP/IP Protocol Framework
Like the seven-layer OSI Reference Model, TCP/IP protocols are based on a layered framework. TCP/IP has four layers, as shown in Figure 2-1. These layers are described in the following sections.
FIGURE 2-1: The four layers of the TCP/IP framework.
Network interface layer
The lowest level of the TCP/IP architecture is the network interface layer. It corresponds to the OSI physical and data link layers. You can use many different TCP/IP protocols at the network interface layer, including Ethernet and token ring for LANs and protocols such as X.25, Frame Relay, and ATM for wide area networks (WANs).
The network interface layer is assumed to be unreliable.
Network layer
The network layer is where data is addressed, packaged, and routed among networks. Several important Internet protocols operate at the network layer:
Internet Protocol (IP): A routable protocol that uses IP addresses to deliver packets to network devices. IP is an intentionally unreliable protocol, so it doesn’t guarantee delivery of information.
Address Resolution Protocol (ARP): Resolves IP addresses to hardware Media Access Control (MAC) addresses, which uniquely identify hardware devices.
Internet Control Message Protocol (ICMP): Sends and receives diagnostic messages. ICMP is the basis of the ubiquitous ping command.
Internet Group Management Protocol (IGMP): Used to multicast messages to multiple IP addresses at once.
Transport layer
The transport layer is where sessions are established and data packets are exchanged between hosts. Two core protocols are found at this layer:
Transmission Control Protocol (TCP): Provides reliable connection-oriented transmission between two hosts. TCP establishes a session between hosts, and then ensures delivery of packets between the hosts.
User Datagram Protocol (UDP): Provides connectionless, unreliable, one-to-one or one-to-many delivery.
Application layer
The application layer of the TCP/IP model corresponds to the session, presentation, and application layers of the OSI Reference Model. A few of the most popular application layer protocols are
Hypertext Transfer Protocol (HTTP): The core protocol of the World Wide Web.
File Transfer Protocol (FTP): A protocol that enables a client to send and receive complete files from a server.
Telnet: The protocol that lets you connect to another computer on the Internet in a terminal emulation mode.
Simple Mail Transfer Protocol (SMTP): One of several key protocols that are used to provide email services.
Domain Name System (DNS): The protocol that allows you to refer to other host computers by using names rather than numbers.
Chapter 3
IP Addresses
IN THIS CHAPTER
One of the most basic components of TCP/IP is IP addressing. Every device on a TCP/IP network must have a unique IP address. In this chapter, I describe the ins and outs of these IP addresses. Enjoy!
Understanding Binary
Before you can understand the details of how IP addressing works, you need to understand how the binary numbering system works because binary is the basis of IP addressing. If you already understand binary, please skip to the section “Introducing IP Addresses.” I don’t want to bore you with stuff that’s too basic.
Counting by ones
Binary is a counting system that uses only two numerals: 0 and 1. In the decimal system (with which most people are accustomed), you use ten numerals: 0–9. In an ordinary decimal number — such as 3,482 — the rightmost digit represents ones; the next digit to the left, tens; the next, hundreds; the next, thousands; and so on. These digits represent powers of ten: first 100 (which is 1); next, 101 (10); then 102 (100); then 103 (1,000); and so on.
In binary, you have only two numerals rather than ten, which is why binary numbers look somewhat monotonous, as
