The DMZ is an expression that originates from the Korean War. It meant a strip of land forcibly kept clear of enemy soldiers. In terms of a firewall, the DMZ is a network that lies between an internal private network and the external public network. DMZ networks are sometimes called perimeter networks. A DMZ is used as an additional buffer to further separate the public network from the internal network.
A gateway is a machine that provides relay services to compensate for the effectsof a filter. The network inhabited by the gateway is often called the DMZ. A gateway in the DMZ is sometimes assisted by an internal gateway. The internal filter is used to guard against the consequences of a compromised gateway, while the outside filter can be used to protect the gateway from attack.
Many firewalls support tri-homing, allowing use of a DMZ network. It is possible for a firewall to accommodate more than three interfaces, each attached to a different network segment.