Internet Control Message Protocol and Destination Host Unreachable

Protocol numbers are used to configure firewalls, routers, and proxy servers. In Internet Protocol version 4 (IPv4, Request for Comments [RFC] 791I), the protocol number can be found in the Protocol field of an IP header.ICMP Echos are used mostly for troubleshooting. When there are 2 hosts which have communication problems, a few simple ICMP Echo requests will show if the 2 hosts have their TCP/IP stacks configured correctly and if there are any problems with the routes packets are taking in order to get to the other side
ICMP Echo Information
Type Name Reference
---- ------------------------- --------- 0 Echo Reply [RFC792]

Codes 0 No Code

1 Unassigned [JBP]

2 Unassigned [JBP]

3 Destination Unreachable [RFC792]

Codes 0 Net Unreachable 1 Host Unreachable 2 Protocol Unreachable 3 Port Unreachable 4 Fragmentation Needed and Don't Fragment was Set 5 Source Route Failed 6 Destination Network Unknown 7 Destination Host Unknown 8 Source Host Isolated 9 Communication with Destination Network is Administratively Prohibited 10 Communication with Destination Host is Administratively Prohibited 11 Destination Network Unreachable for Type of Service 12 Destination Host Unreachable for Type of Service 13 Communication Administratively Prohibited [RFC1812] 14 Host Precedence Violation [RFC1812] 15 Precedence cutoff in effect [RFC1812]

4 Source Quench [RFC792] Codes 0 No Code

5 Redirect