Along with ICMP for IPv4, ICMPv6 is used to communicate updates or error messages and has its own set of types to identify messages. Because ICMPv6 provides additional information on IPv6 router and host configuration, you'll find specific type values that help provide this information.
A shortlist of ICMPv6 type values includes the following:
- Type 1: Destination unreachable
- Type 2: Packet too big
- Type 3: Time exceeded
- Type 4: Parameter problem
- Type 130: Multicast listener query
- Type 131: Multicast listener report
In some cases, the type will have a corresponding code value to further define the message. If the type does not have a corresponding code value, the code value will be set to 0, as shown in the ICMPv6 multicast listener report screenshot.
Let's look at the following examples.
Type 1 (destination unreachable) has several code values. Some of them are as follows:
- Code 0: No route to destination
- Code 1: Communication with destination administratively prohibited
- Code 2: Beyond the scope of the source address
- Code 3: Address unreachable
Type 3 (time exceeded) has two codes, as follows:
- Code 0: Hop limit exceeded in transit
- Code 1: Fragment reassembly time exceeded
As we have learned, ICMP headers hold a value for Type and Code to convey information on what is happening on the network. However, some of the ICMP types are no longer used since, over time, they have been found to be ineffective and are considered deprecated, such as these examples:
- Type 33: IPv6 Where-are-you (deprecated)
- Type 34: IPv6 I-am-here (deprecated)
- Type 35: Mobile registration request (deprecated)
Now that you have seen how ICMP works, you should also see that ICMP can provide a great deal of information on a network and devices. As a result, it's important to understand that this protocol can be used in malicious ways, and that the firewall rules should be tuned to prevent malicious activity, as outlined in the next section.