Everyone is familiar with Internet Protocol (IP), which is responsible for routing and addressing traffic. However, many people are not familiar with Internet Control Message Protocol (ICMP), the unsung hero of the network layer. ICMP is a powerful protocol that helps IP do its job. 

In this chapter, we'll learn about ICMP, which is the sister protocol to IP and works in the network layer of the OSI model. First, we'll go through an overview, so that you have a general understanding of the main functions of ICMP. We will then evaluate both ICMP (used with IPv4) and ICMPv6 (used with IPv6) so that you can compare some of the main differences.

In addition, you'll get a better understanding of the two types of messages: error reporting and queries. We will look at common type and code values, as well as some basic firewall guidelines in terms of what types of ICMP messages to allow on your network. At the end of the chapter, you'll see how ICMP is the scout for IP and plays a major role in delivering data.

This chapter will address all of this by covering the following:

• Understanding the purpose of ICMP
• Dissecting ICMPv4 and ICMPv6
• Sending ICMP messages
• Evaluating type and code values
• Configuring firewall rules