If I am correct, a gateway device has several network interfaces.

Is a gateway a bi-directional or uni-directional concept? In other words, is a gateway for bidrectional communication between two networks, or can be just for communication from just one network to the other network? Does that depend on what direction(s) the routing table on the gateway allows?



Network components are generally bidirectional. A gateway is commonly used for routing in both directions, but not necessarily so (with asymmetric routing).

For each entry, the routing table points in a specific direction for outbound traffic. For each destination route, the table indicates the next-hop gateway and (most often) the interface to be used. For unnumbered links, only the interface is indicated.

  • Thanks. When people say a gateway, is it "a gateway of a network" or "a gateway between two or more networks" (implying belonging to all the networks which the gateway is connected to).
    – Tim
    Mar 27 '19 at 13:37
  • A gateway generally is connected to two or more networks - wouldn't work as a gateway otherwise.
    – Zac67
    Mar 27 '19 at 13:42
  • That is correct. Let me try to rephrase my questions. Can a device be listed as gateway in the routing tables of hosts in one network, but not in the other network? See networkengineering.stackexchange.com/questions/58060/…
    – Tim
    Mar 27 '19 at 13:53
  • @Tim The routing table lists a device's IP address, not the device itself. Since a gateway needs to be part of the local network, a router can never use the same IP address on different networks. Whether or not a different interface/IP from the same device is a gateway on a remote network doesn't matter (it usually is though).
    – Zac67
    Mar 27 '19 at 15:58

Generally a gateway defines as L3 interfàce Configured on L3 device . Traffic reaching this gateway can be bidirectional , but subjected to access-list , route entries towards destinations if destionation is on different networks.


A Security Gateway may be bi-directional or unidirectional. In the context of Cross Domain Solutions commonly found in government, military and critical infrastructure architectures one refers to a data didode as an unidirectional gateway and a bidirectional Gateway which employs unique security functions as a bidirectional security Gateway. The latter terminates all traffic within the Gateway to filter the data and restablish a connection from a unique IP on the Gateway side to a specific IP on the LOW side.

  • 1
    Don't think that's what the OP meant. Removed potentially promotional link.
    – Zac67
    Jan 18 at 20:55

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