Represents a particular forwarding. From RFC 4254, s. 7.1
The 'address to bind' and 'port number to bind' specify the IP address (or domain name) and port on which connections for forwarding are to be accepted. Some strings used for 'address to bind' have special-case semantics. o "" means that connections are to be accepted on all protocol families supported by the SSH implementation. o "0.0.0.0" means to listen on all IPv4 addresses. o "::" means to listen on all IPv6 addresses. o "localhost" means to listen on all protocol families supported by the SSH implementation on loopback addresses only ([RFC3330] and [RFC3513]). o "127.0.0.1" and "::1" indicate listening on the loopback interfaces for IPv4 and IPv6, respectively.