When a packet is undeliverable, a Destination Unreachable ICMP Type 3 message is generated. Type 3 ICMP packets can have a code value of 0 to 15.

The following ICMP Type 3 codes values are defined in RFC 1812:

Hexadecimal Decimal Description
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0x00 0 Network Unreachable - generated by a router if a forwarding path
(route) to the destination network is not available;

0x01 1 Host Unreachable - generated by a router if a forwarding path
(route) to the destination host on a directly connected network
is not available (does not respond to ARP);

0x02 2 Protocol Unreachable - generated if the transport protocol
designated in a datagram is not supported in the transport layer
of the final destination;

0x03 3 Port Unreachable - generated if the designated transport protocol
(e.g., UDP) is unable to demultiplex the datagram in the
transport layer of the final destination but has no protocol
mechanism to inform the sender;

0x04 4 Fragmentation Needed and DF Set - generated if a router needs to
fragment a datagram but cannot since the DF flag is set;

0x05 5 Source Route Failed - generated if a router cannot forward a
packet to the next hop in a source route option;

0x06 6 Destination Network Unknown - This code SHOULD NOT be generated
since it would imply on the part of the router that the
destination network does not exist (net unreachable code 0
SHOULD be used in place of code 6);

0x07 7 Destination Host Unknown - generated only when a router can
determine (from link layer advice) that the destination host
does not exist;

0x0B 11 Network Unreachable For Type Of Service - generated by a router
if a forwarding path (route) to the destination network with the
requested or default TOS is not available;

0x0C 12 Host Unreachable For Type Of Service - generated if a router
cannot forward a packet because its route(s) to the destination
do not match either the TOS requested in the datagram or the
default TOS (0).

0x0D 13 Communication Administratively Prohibited - generated if a
router cannot forward a packet due to administrative filtering;

0x0E 14 Host Precedence Violation. Sent by the first hop router to a
host to indicate that a requested precedence is not permitted
for the particular combination of source/destination host or
network, upper layer protocol, and source/destination port;

0x0F 15 Precedence cutoff in effect. The network operators have imposed
a minimum level of precedence required for operation, the
datagram was sent with a precedence below this level;