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This virtualisation makes multiple concurrent connections on a single network interface possible. You could say that a port is a virtualised endpoint. The purpose of ports is to differentiate multiple endpoints on a given network address. Note that address/port does not completely identify a socket (more on this later). This usage originates with RFC793, the original TCP specification.Ī TCP connection is defined by two endpoints aka sockets.Īn endpoint (socket) is defined by the combination of a network address and a port identifier. The combination of an IP address and a port is strictly known as an endpoint and is sometimes called a socket. The OSI model is all very well but has never been completely implemented, much less widely deployed in high-traffic high-stress conditions. Although not used in a networking context, the French word porte literally means door or gateway, further emphasising the fact that ports are transportation endpoints whether you ship data or big steel containers.įor the purpose of this discussion I will limit consideration to the context of TCP-IP networks.
In the broadest possible sense, a port is a point of ingress or egress. Nevertheless, in network parlance the word "socket" carries so much baggage that a careful re-examination is necessary. You'd think a name like "socket" would be self-explanatory: it was obviously chosen to evoke imagery of the endpoint into which you plug a network cable, there being strong functional parallels. This was an interesting question that forced me to re-examine a number of things I thought I knew inside out. There can only be one listener socket for a given address/port combination. There can be concurrent connections to a service endpoint, because a connection is identified by both its local and remote endpoints, allowing traffic to be routed to a specific service instance. A TCP socket is an endpoint instance defined by an IP address and a port in the context of either a particular TCP connection or the listening state.Ī port is a virtualisation identifier defining a service endpoint (as distinct from a service instance endpoint aka session identifier).Ī TCP socket is not a connection, it is the endpoint of a specific connection.