Gateway (telecommunications)


In a communications network, a network node equipped for interfacing with another network that uses different protocols.
A gateway may contain devices such as protocol translators, impedance matching devices, rate converters, fault isolators, or signal translators as necessary to provide system interoperability. It also requires the establishment of mutually acceptable administrative procedures between both networks.
A protocol translation/mapping gateway interconnects networks with different network protocol technologies by performing the required protocol conversions.
Loosely, a computer is configured to perform the tasks of a gateway. For a specific case, see default gateway.
Routers exemplify special cases of gateways.

Gateways, also called protocol converters, can operate at any layer of the OSI model. The job of a gateway is much more complex than that of a router or switch. Typically, a gateway must convert one protocol stack into another.

Details
Gateways work on all seven OSI layers . The main job of a gateway is to convert protocols among communications networks. A router by itself transfers, accepts and relays packets only across networks using similar protocols. A gateway on the other hand can accept a packet formatted for one protocol (e.g. AppleTalk) and convert it to a packet formatted for another protocol (e.g. TCP/IP) before forwarding it. A gateway can be implemented in hardware, software or both, but they are usually implemented by software installed within a router. A gateway must understand the protocols used by each network linked into the router. Gateways are slower than bridges, switches and (non-gateway) routers.
A gateway is a network point that acts as an entrance to another network. On the Internet, a node or stopping point can be either a gateway node or a host (end-point) node. Both the computers of Internet users and the computers that serve pages to users are host nodes, while the nodes that connect the networks in between are gateways. For example, the computers that control traffic between company networks or the computers used by internet service providers (ISPs) to connect users to the internet are gateway nodes.

In the network for an enterprise, a computer server acting as a gateway node is often also acting as a proxy server and a firewall server. A gateway is often associated with both a router, which knows where to direct a given packet of data that arrives at the gateway, and a switch, which furnishes the actual path in and out of the gateway for a given packet.
On an IP network, clients should automatically send IP packets with a destination outside a given subnet mask to a network gateway. A subnet mask defines the IP range of a network. For example, if a network has a base IP address of 192.168.0.0 and has a subnet mask of 255.255.255.0, then any data going to an IP address outside of 192.168.0.X will be sent to that network's gateway. While forwarding an IP packet to another network, the gateway might or might not perform Network Address Translation.

A gateway is an essential feature of most routers, although other devices (such as any PC or server) can function as a gateway.
Most computer operating systems use the terms described above. A computer running Microsoft Windows however describes this standard networking feature as Internet Connection Sharing; which will act as a gateway, offering a connection between the Internet and an internal network. Such a system might also act as a DHCP server. Dynamic Host Configuration Protocol (DHCP) is a protocol used by networked devices (clients) to obtain various parameters necessary for the clients to operate in an Internet Protocol (IP) network. By using this protocol, system administration workload greatly decreases, and devices can be added to the network with minimal or no manual configurations.

Examples

A very popular example is connecting a Local Area Network or Wireless LAN to the Internet or other Wide Area Network. In this case the gateway connects an IPX/SPX (the LAN) to a TCP/IP network (the Internet).
MainWay is the Bull brand for a gateway which connects DSA to TCP/IP telecom

Connecting IP Networks
Gateways that connect two IP-based networks, have two IP addresses, one on each network. A gateway address like 192.168.1.1 is a Private address, and is the address to which traffic is sent from the LAN. The other IP address is the Wide Area Network address, this is the address to which traffic is sent coming from the WAN. When this is the Internet, that address is usually assigned by an ISP.
When talking about the gateway IP address, commonly the LAN-address of the gateway is meant.
If private addressing is used then the addresses of computers connected to the LAN are hidden behind the WAN gateway. That is, remote computers located "out there" on the WAN can only communicate with LAN stations via the gateway's WAN IP address. To regulate traffic between the WAN and the LAN, the gateway commonly performs Network Address Translation (NAT), presenting all of the LAN traffic to the WAN as coming from the gateway's WAN IP address and doing packet sorting and distribution of return WAN traffic to the local network.

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