A-2. ICMP Message Types
Internet Control Message Protocol (ICMP) is used to transport error or control messages between routers and other devices. An ICMP message is encapsulated as the payload in an IP packet, as shown in Figure A-2. This information appears immediately following the IP header. Many of the ICMP message types also have a code number that can be used. The code field further specifies how the message type should be applied when it is received. Cisco firewalls can't use the code field in access lists, so that information is not presented here.
Figure A-2. ICMP Message Format
Notice that in the case of an error condition, the first ...
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