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Cisco IOS in a Nutshell, 2nd Edition by James Boney

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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 ...

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