TCP/IP Basics You Should Know to Get a Networking Job

By Peter H. Gregory, Bill Hughes

If you hope get a job in networking, you will want to be familiar with TCP/IP. Developed in the 1970s as a robust military communications network that had some self-healing properties and resilience, TCP/IP has formed the basis for virtually every home, business, and commercial network, as well as the global Internet itself.

TCP/IP is a packet-based technology in which messages are bundled into packets that are routed to their destinations. A single packet has a source address, a destination address, a protocol number, and payload (the contents of a message).

TCP/IP addressing

The source address and destination address follow a numbering convention, with a global authority that assigns addresses to organizations. In TCP/IP version 4, the form of an address is

xxx.xxx.xxx.xxx

where each xxx (octet) is commonly portrayed as a decimal value and can range from 1 through 255. In TCP/IP version 6, the form of an address is

xxxx:xxxx:xxxx:xxxx:xxxx:xxxx:xxxx:xxxx

where each xxxx (hextet) is a hexadecimal value that can range from 0000 through FFFF.

TCP/IP routing

In TCP/IP, routers process packets as they make their way from their source to their destination. Think of a router as a traffic cop in a street intersection who tells you which way to turn. A router examines a packet’s destination address, consults its routing table, and then sends the packet in the correct direction to get it closer to its ultimate destination.

Routers exchange routing information so that each router has a better idea about which direction to send each packet. They exchange this information through several routing protocols, such as RIP, BGP, IGRP, OSPF, or IS-IS. These routing protocols each have best uses; some are used by Internet backbone routers, and others are better suited for routers inside a company. Some become obsolete as newer and better ones are developed.

TCP/IP protocols

TCP/IP has two basic protocols, on top of which nearly all the others are used through encapsulation. These two basic protocols are

  • UDP: Formally known as User Datagram Protocol, UDP is a simple connectionless protocol typically used to send a message in a single packet.

  • TCP (Transmission Control Protocol): This connection-oriented protocol is usually intended for a longer conversation between systems. A TCP session is established by something called a three-way handshake, which works something like this:

Station A: Hello, I’m Station A and I’d like to talk to you, Station B.
Station B: Hello, I’m Station B and yes I’d like to talk to you, Station A.
Station A: Hello Station B, I’m happy we have agreed to talk to each other.

The TCP and UDP protocols contain hundreds of established protocol standards, a few of which are well known and frequently used, such as

  • HTTP (HyperText Transport Protocol): Using port 80, transports user web traffic without encryption.

  • HTTPS (HyperText Transport Protocol Secure): Using port 443, transports web traffic with encryption.

  • SMTP (Simple Mail Transport Protocol): Using port 25, transports email without encryption.

  • DNS (Domain Name Service) protocol: Using port 43 (on both TCP and UDP services), translates domain names such as www.dummies.com into IP addresses.

  • FTP (File Transfer Protocol): Using ports 20 and 21, enables bulk file transfers without encryption.

  • SSH (Secure SHell) protocol: Using port 22, provides administrative access to systems and network devices.

To understand concepts such as encapsulation, you should become familiar with the OSI Network Model.