The ABCs of TCP/IP, 2nd Edition

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Appendix A

ICMP Type and Code Values

This appendix provides a listing of ICMP Type and Code values. One can use

the information in this appendix when configuring router access lists or a

firewall that performs a packet filtering operation. For example, if one creates

a router access list that only allows ICMP Type 0 (Echo Reply) messages in

the inbound direction from an Internet connection, in effect one permits

responses to echo messages originated by users behind the router but blocks

all other ICMP messages. Thus, this would allow one’s employees to ping

distant hosts while blocking users on the Internet from pinging hosts on one’s

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