Cisco IOS in a Nutshell, 2nd Edition

Appendix A. Appendix: Network Basics

Throughout this book, I use IP address notation without much explanation. I take for granted that you understand the meaning of 192.168.0.1/24. And most likely, you probably do. However, I find that even the seasoned professional can sometimes forget the simplest information. This appendix serves as a useful reminder in those moments of brain fog when the network basics elude you. In addition to IP address notation and subnetting, this appendix reviews the OSI reference model and provides some details about IPv6.

IPv4 Subnetting

Back in the early days of the Internet, there were three major classes of addressing that were labeled A through C. Table A-1 shows how many hosts the Internet could support based on this system.

Table A-1. Original IPv4 class system

	Networks per class	Hosts per network	IP range
Class A	126	16,777,214	1.0.0.0-127.255.255.255
Class B	65,000	65,534	128.0.0.0-191.255.255.255
Class C	2,000,000	254	192.0.0.0-223.255.255.255

Tip

Note: Two other classes, D and E, were reserved for multicast and “future use,” respectively.

With the growth of the Internet, these classes became obsolete for a few reasons, the biggest of which was address space depletion. In the early 1990s, we could see that we would run out of address space to give to new Internet users. Another problem was that the routing tables on the Internet backbone routers were growing out of control. Without the ability to summarize routing information, the ...