| ICMPv4 | ICMPv40 | (type) | Echo Reply Reply to an Echo Request; this is what ping receives back. RFC 792 | RFC 792 | — |
| ICMPv4 | ICMPv43 | (type) | Destination Unreachable A router or the destination host could not deliver the datagram. The code says why, and the message carries the start of the original packet. RFC 792 | RFC 792 | — |
| ICMPv4 | ICMPv43 / 0 | 0 | Net Unreachable A router has no route to the destination network. RFC 792 | RFC 792 | — |
| ICMPv4 | ICMPv43 / 1 | 1 | Host Unreachable The destination host could not be reached, for example when the last router cannot deliver to it on its link. RFC 792 | RFC 792 | — |
| ICMPv4 | ICMPv43 / 2 | 2 | Protocol Unreachable The destination host does not support the protocol named in the IP Protocol field. RFC 792 | RFC 792 | — |
| ICMPv4 | ICMPv43 / 3 | 3 | Port Unreachable Nothing is listening on the destination port, for example a UDP datagram sent to a closed port. RFC 792 | RFC 792 | — |
| ICMPv4 | ICMPv43 / 4 | 4 | Fragmentation Needed and Don't Fragment was Set The datagram is larger than the next link's MTU but has Don't Fragment set. Since RFC 1191 the message includes the next-hop MTU, and Path MTU Discovery depends on it. RFC 792, RFC 1191 | RFC 792, RFC 1191 | — |
| ICMPv4 | ICMPv43 / 5 | 5 | Source Route Failed A router could not forward the datagram to the next hop in its source route option. RFC 792 | RFC 792 | — |
| ICMPv4 | ICMPv43 / 6 | 6 | Destination Network Unknown Claims the destination network does not exist. RFC 1812 says routers should not send it and should use code 0 instead. RFC 1122 | RFC 1122 | — |
| ICMPv4 | ICMPv43 / 7 | 7 | Destination Host Unknown The destination host does not exist, as a router determined from link-layer information. RFC 1122 | RFC 1122 | — |
| ICMPv4 | ICMPv43 / 8 | 8 | Source Host Isolated The source host is isolated. RFC 1812 says routers should not send it and should use code 0 or 1 instead. RFC 1122 | RFC 1122 | — |
| ICMPv4 | ICMPv43 / 9 | 9 | Communication with Destination Network is Administratively Prohibited Communication with the destination network is administratively prohibited. It was meant for U.S. military encryption devices; RFC 1812 says filtering routers should use code 13 instead. RFC 1122 | RFC 1122 | — |
| ICMPv4 | ICMPv43 / 10 | 10 | Communication with Destination Host is Administratively Prohibited Communication with the destination host is administratively prohibited. It was meant for U.S. military encryption devices; RFC 1812 says filtering routers should use code 13 instead. RFC 1122 | RFC 1122 | — |
| ICMPv4 | ICMPv43 / 11 | 11 | Destination Network Unreachable for Type of Service The destination network is unreachable for the requested Type of Service. RFC 1122 | RFC 1122 | — |
| ICMPv4 | ICMPv43 / 12 | 12 | Destination Host Unreachable for Type of Service The destination host is unreachable for the requested Type of Service. RFC 1122 | RFC 1122 | — |
| ICMPv4 | ICMPv43 / 13 | 13 | Communication Administratively Prohibited A router could not forward the packet because of administrative filtering. Routers may be configured not to send it, so a filter can also drop packets silently. RFC 1812 | RFC 1812 | — |
| ICMPv4 | ICMPv44 | (type) | Source Quench (deprecated by RFC 6633) Formerly asked the sender to slow down. RFC 6633 deprecates it: hosts must not send it, and receivers may silently discard it. RFC 792, RFC 6633 | RFC 792, RFC 6633 | — |
| ICMPv4 | ICMPv45 | (type) | Redirect A router tells a host on the same network to use a different next-hop router for some traffic. RFC 792 | RFC 792 | — |
| ICMPv4 | ICMPv45 / 0 | 0 | Redirect Datagram for the Network Redirect for a whole destination network. RFC 1812 forbids routers from sending this code; they send host redirects (code 1). RFC 792 | RFC 792 | — |
| ICMPv4 | ICMPv45 / 1 | 1 | Redirect Datagram for the Host Redirect for one destination host: send its traffic to the indicated router. RFC 792 | RFC 792 | — |
| ICMPv4 | ICMPv45 / 2 | 2 | Redirect Datagram for the Type of Service and Network Redirect for a destination network and Type of Service. RFC 1812 forbids routers from sending this code. RFC 792 | RFC 792 | — |
| ICMPv4 | ICMPv45 / 3 | 3 | Redirect Datagram for the Type of Service and Host Redirect for a destination host and Type of Service. RFC 792 | RFC 792 | — |
| ICMPv4 | ICMPv48 | (type) | Echo Request Asks the destination to send back an Echo Reply; ping sends this. RFC 792 | RFC 792 | — |
| ICMPv4 | ICMPv49 | (type) | Router Advertisement A router announces its addresses on the link so hosts can find a default router (ICMP Router Discovery, RFC 1256). RFC 1256 | RFC 1256 | — |
| ICMPv4 | ICMPv410 | (type) | Router Solicitation A host asks routers on the link to send a Router Advertisement right away (RFC 1256). RFC 1256 | RFC 1256 | — |
| ICMPv4 | ICMPv411 | (type) | Time Exceeded The datagram was discarded because a time limit ran out: the TTL reached zero, or fragment reassembly took too long. RFC 792 | RFC 792 | — |
| ICMPv4 | ICMPv411 / 0 | 0 | Time to Live Exceeded in Transit A router discarded the datagram because its TTL reached zero. traceroute relies on this message. RFC 792 | RFC 792 | — |
| ICMPv4 | ICMPv411 / 1 | 1 | Fragment Reassembly Time Exceeded The destination gave up reassembling a fragmented datagram because not all fragments arrived in time. RFC 792 | RFC 792 | — |
| ICMPv4 | ICMPv412 | (type) | Parameter Problem A host or router found a problem in the IP header and discarded the datagram. RFC 792 | RFC 792 | — |
| ICMPv4 | ICMPv412 / 0 | 0 | Pointer indicates the error The Pointer field gives the byte offset of the problem in the original header. RFC 792 | RFC 792 | — |
| ICMPv4 | ICMPv412 / 1 | 1 | Missing a Required Option A required option is missing. It was defined by RFC 1108 (now Historic) for the IP security option. RFC 1108 | RFC 1108 | — |
| ICMPv4 | ICMPv412 / 2 | 2 | Bad Length The header or datagram length is invalid. RFC 792 | RFC 792 | — |
| ICMPv4 | ICMPv413 | (type) | Timestamp Asks the destination for its time-of-day timestamps. RFC 792 | RFC 792 | — |
| ICMPv4 | ICMPv414 | (type) | Timestamp Reply Reply to a Timestamp request, with the times the request was received and the reply sent. RFC 792 | RFC 792 | — |
| ICMPv6 | ICMPv61 | (type) | Destination Unreachable The packet could not be delivered for a reason other than congestion; the code says why. RFC 4443 · error | RFC 4443 | error |
| ICMPv6 | ICMPv61 / 0 | 0 | No route to destination A router has no matching route for the destination (it has no default route either). RFC 4443 · error | RFC 4443 | error |
| ICMPv6 | ICMPv61 / 1 | 1 | Communication with destination administratively prohibited Delivery is administratively prohibited, for example by a firewall filter. RFC 4443 · error | RFC 4443 | error |
| ICMPv6 | ICMPv61 / 2 | 2 | Beyond scope of source address The destination is beyond the scope of the source address, for example a link-local source with a global destination. RFC 4443 · error | RFC 4443 | error |
| ICMPv6 | ICMPv61 / 3 | 3 | Address unreachable Delivery failed for another reason, such as failing to resolve the destination's link-layer address with Neighbor Discovery. RFC 4443 · error | RFC 4443 | error |
| ICMPv6 | ICMPv61 / 4 | 4 | Port unreachable The transport protocol has no listener on the destination port, for example UDP. RFC 4443 · error | RFC 4443 | error |
| ICMPv6 | ICMPv61 / 5 | 5 | Source address failed ingress/egress policy The source address is not allowed by an ingress or egress filtering policy. RFC 4443 · error | RFC 4443 | error |
| ICMPv6 | ICMPv61 / 6 | 6 | Reject route to destination The packet matched a reject route for the destination. RFC 4443 · error | RFC 4443 | error |
| ICMPv6 | ICMPv62 | (type) | Packet Too Big The packet is larger than the next link's MTU; the message carries that MTU. IPv6 routers do not fragment, so Path MTU Discovery depends on it. RFC 4443 · error | RFC 4443 | error |
| ICMPv6 | ICMPv63 | (type) | Time Exceeded The packet was discarded because a limit ran out: the Hop Limit reached zero, or fragment reassembly timed out. RFC 4443 · error | RFC 4443 | error |
| ICMPv6 | ICMPv63 / 0 | 0 | Hop limit exceeded in transit A router discarded the packet because its Hop Limit reached zero. traceroute relies on this message. RFC 4443 · error | RFC 4443 | error |
| ICMPv6 | ICMPv63 / 1 | 1 | Fragment reassembly time exceeded The destination gave up reassembling a fragmented packet because not all fragments arrived in time. RFC 4443 · error | RFC 4443 | error |
| ICMPv6 | ICMPv64 | (type) | Parameter Problem A field in the IPv6 header or an extension header could not be processed; the Pointer gives its offset. RFC 4443 · error | RFC 4443 | error |
| ICMPv6 | ICMPv64 / 0 | 0 | Erroneous header field encountered A header field has a value that cannot be processed. RFC 4443 · error | RFC 4443 | error |
| ICMPv6 | ICMPv64 / 1 | 1 | Unrecognized Next Header type encountered A Next Header value was not recognized. RFC 4443 · error | RFC 4443 | error |
| ICMPv6 | ICMPv64 / 2 | 2 | Unrecognized IPv6 option encountered An IPv6 option was not recognized, and its option type asks for this error to be reported. RFC 4443 · error | RFC 4443 | error |
| ICMPv6 | ICMPv6128 | (type) | Echo Request Asks the destination to send back an Echo Reply; ping sends this. RFC 4443 · informational | RFC 4443 | informational |
| ICMPv6 | ICMPv6129 | (type) | Echo Reply Reply to an Echo Request; this is what ping receives back. RFC 4443 · informational | RFC 4443 | informational |
| ICMPv6 | ICMPv6130 | (type) | Multicast Listener Query A router asks which multicast addresses have listeners on the link (MLD). RFC 2710 · informational | RFC 2710 | informational |
| ICMPv6 | ICMPv6131 | (type) | Multicast Listener Report A node reports that it listens to a multicast address (MLDv1). RFC 2710 · informational | RFC 2710 | informational |
| ICMPv6 | ICMPv6132 | (type) | Multicast Listener Done A node reports that it stopped listening to a multicast address (MLDv1). RFC 2710 · informational | RFC 2710 | informational |
| ICMPv6 | ICMPv6133 | (type) | Router Solicitation A host asks routers on the link to send a Router Advertisement right away (Neighbor Discovery). RFC 4861 · informational | RFC 4861 | informational |
| ICMPv6 | ICMPv6134 | (type) | Router Advertisement A router announces itself and link parameters such as prefixes; hosts use it for default routes and address autoconfiguration (SLAAC). RFC 4861 · informational | RFC 4861 | informational |
| ICMPv6 | ICMPv6135 | (type) | Neighbor Solicitation Asks a neighbor for its link-layer address, checks that it is still reachable, or detects duplicate addresses (DAD). RFC 4861 · informational | RFC 4861 | informational |
| ICMPv6 | ICMPv6136 | (type) | Neighbor Advertisement Answers a Neighbor Solicitation, or announces unsolicited that a link-layer address changed. RFC 4861 · informational | RFC 4861 | informational |
| ICMPv6 | ICMPv6137 | (type) | Redirect Message A router tells a host about a better first hop for a destination, or that the destination is on the same link. RFC 4861 · informational | RFC 4861 | informational |
| ICMPv6 | ICMPv6143 | (type) | Multicast Listener Discovery Version 2 (MLDv2) Report A node reports its multicast listening state, including source filters (MLDv2, RFC 3810). RFC 3810 · informational | RFC 3810 | informational |