7 IPv4 addressing

In chapter 6, we focused on Layer 2 of the TCP/IP model: how switches use information in the Ethernet header to make forwarding decisions. In this chapter, we will move up a layer to Layer 3 and look at the contents of the Internet Protocol version 4 (IPv4) header, focusing on IPv4 addressing.

We are now in the realm of routers, rather than switches. Whereas switches use information in the Layer 2 header to decide how to forward messages to their proper destinations, routers use information in the Layer 3 header to make their forwarding decisions. In this chapter, we won’t yet focus on exactly how routers make those forwarding decisions; we will leave that for part 2 of this book. Instead, we will first focus on the contents of the IPv4 header and the addresses used in that header.

The specific exam topic we will cover is topic 1.6: Configure and verify IPv4 addressing and subnetting. However, IPv4 addressing is not only relevant to exam topic 1.6; it is a fundamental topic that is essential to understanding nearly any other CCNA exam topic. Also note that we will cover subnetting, the second half of topic 1.6, in part 2 of this book.

7.1 The IPv4 header

7.1.1 The Version field

7.1.2 The IHL field

7.1.3 The DSCP and ECN fields

7.1.4 The Total Length field

7.1.5 The Identification, Flags, and Fragment Offset fields

7.1.6 The TTL field

7.1.7 The Protocol field

7.1.8 The Header Checksum field

7.1.9 The Source Address and Destination Address fields

7.1.10 The Options field

7.2 The binary number system

7.2.1 Decimal

7.2.2 Binary

7.3 IPv4 addressing

7.3.1 The structure of an IPv4 address

7.3.2 Configuring IPv4 addresses on a router

7.3.3 Attributes of an IPv4 network