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How to become pingable behind a router/firewall ?

Ping (an echo request) is a computer network administration utility used to test the reachability of a host on an Internet Protocol (IP) network and to measure the round-trip time. Some software firewalls, as well as NAT routers may block ping requests in an effort to "hide" your external IP address and prevent any response to potential intruders and prevent possible denial-of-service attacks in the form of a ping flood, in which an attacker overwhelms the victim with ICMP echo request packets.

In a broader term, ping can also be used to describe the transmission of any message for the purpose of locating or testing/identifying a network device and measuring latency. Such pings may not necessarily use ICMP packets, another common practice is using the UDP (User Datagram Protocol). A common use for identification purposes using TCP/UDP instead of ICMP is port 113 (ident)

A number of legitimate programs, such as IRC, online games, and network tools may need your computer to be "pingable". To accomplish this, you will have to determine first what device/software on your LAN is blocking ping requests. Commonly, this would be a hardware router/modem, or it could be a software firewall.

To enable echo reqests (ping) behind a NAT router, you must log into the router and explicitly configure it to respond to ICMP echo requests on its WAN port. We have links to manuals of over 2500 common routers/firewalls listed in our hardware database.

Most computers will, by default, automatically respond to ICMP echo request (pings). If you can't ping a machine, it is likely that it is behind a NAT router or some sort of firewall.
Many routers/gateways block ICMP echo requests (or simply drop ICMP when under high load).
ICMP is a session-less protocol and does not use "ports", they are a construct used by some protocols, such as TCP/UDP to maintain a persistent connection between computers.
Under Unix-like OSes, the traceroute utility uses UDP by default, while under Windows tracert uses ICMP.

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