ICMP Codes

Type

Name

Description

3

Destination Unreachable Codes

Net Unreachable

The sending device knows about the network but believes it is not available at this time. Perhaps the network is too far away through the known route.

Host Unreachable

The sending devices knows about host but doesn't get ARP reply, indicating the host is not available at this time

Protocol Unreachable

The protocol defined in IP header cannot be forwarded.

Port Unreachable

The sending device does not support the port number you are trying to reach

Fragmentation Needed and Don't Fragment was Set

The router needs to fragment the packet to forward it across a link that supports a smaller maximum transmission unit (MTU ) size. However, application set the Don't Fragment bit.

Source Route Failed

ICMP sender can't use the strict or loose source routing path specified in the original packet.

Destination Network Unknown

ICMP sender does not have a route entry for the destination network, indicating this network may never have been an available.

Destination Host Unknown

ICMP sender does not have a host entry, indicating the host may never have been available on connected network.

Source Host Isolated

ICMP sender (router) has been configured to not forward packets from source (the old electronic pink slip).

Communication with Destination Network is Administratively Prohibited

ICMP sender (router) has been configured to block access to the desired destination network.

Communication with Destination Host is Administratively Prohibited

ICMP sender (router) has been configured to block access to the desired destination host.

Destination Network Unreachable for Type of Service

The sender is using a Type of Service (TOS) that is not available through this router for that specific network.

Destination Host Unreachable for Type of Service

The sender is using a Type of Service (TOS) that is not available through this router for that specific host.

Communication Administratively Prohibited

ICMP sender is not available for communications at this time.

Host Precedence Violation

Precedence value defined in sender's original IP header is not allowed (for example, using Flash Override precedence).

5

Redirect Codes

Redirect Datagram for the Network (or subnet)

ICMP sender (router) is not the best way to get to the desired network. Reply contains IP address of best router to destination. Dynamically adds a network entry in original sender's routing tables.

Redirect Datagram for the Host

ICMP sender (router) is not the best way to get to the desired host. Reply contains IP address of best router to destination. Dynamically adds a host entry in original sender's route tables.

Redirect Datagram for Type of the Service and Network

ICMP sender (router) does not offer a path to the destination network using the TOS requested. Dynamically adds a network entry in original sender's route tables.

Redirect Datagram for the Type of Service and Host

ICMP sender (router) does not offer a path to the destination host using the TOS requested. Dynamically adds a host entry in original sender's route tables.

6

Alternate Host Address Codes

Alternate Address for Host

Reply that indicates another host address should be used for the desired service. Should redirect application to another host.

11

Time Exceeded Codes

Time to Live exceeded in Transit

ICMP sender (router) indicates that originator's packet arrived with a Time To Live (TTL) of 1. Routers cannot decrement the TTL value to 0 and forward the packet.

Fragment Reassembly Time Exceeded

ICMP sender (destination host) did not receive all fragment parts before the expiration (in seconds of holding time) of the TTL value of the first fragment received.

12

Parameter Problem Codes

Pointer indicates the error

Error is defined in greater detail within the ICMP packet.

Missing a Required Option

ICMP sender expected some additional information in the Option field of the original packet.

Bad Length

Original packet structure had an invalid length.