Unix Power Tools, 3rd Edition

Chapter 46. Connectivity

TCP/IP — IP Addresses and Ports

TCP/IP networking is a part of the Open Systems Interconnection (OSI) Model. Much like you can string together lots of little single-purpose Unix tools to do complex tasks, the OSI Model is made up of specific single-purpose layers that work together. Each layer builds on the layers below. Layers 1 and 2 are concerned with hardware; physical standards such as required voltages and low-level protocols like Ethernet reside there. Layers 3 and 4 are networking layers, which this article introduces. Layers 5 through 7 are application layers, where networking interfaces such as BSD sockets and applications such as web browsers, telnet clients, and diagnostic tools live.

For most Unixes, the fundamentals of networking (once you get past the network device drivers) are the Layer 3 Internet Protocol (IP) and a Layer 4 protocol on top of it, either the Transport Control Protocol (TCP), the User Datagram Protocol (UDP), or the IP Control Message Protocol (ICMP). These four protocols are so commonly treated as one unit that you’ll often see them referred to together as TCP/IP.

Internet Protocol (IP)

IP’s job is to get small chunks of data, called packets, from one machine to another. It is a “best effort” protocol; that is, it makes its best effort to deliver each packet to the right host, and if it can’t, it simply drops the packet on the floor. It may seem like losing bits of your data would be a bad thing, but it turns out that ...