An Expressway Server has both network protocols (IPv4 and IPv6) enabled.

If the calling endpoint is on an IPv4-Only network and the called endpoint is on an IPV6-Only network, a NAT64 device must convert the IPv4 addresses to IPv6 addresses.

Endpoints register with either IPv4 or IPv6 addresses, but dual-stack endpoints choose IPv6 protocol by default.

If the calling endpoint is on an IPv4-Only network and the called endpoint is on an IPv6-Only network, the Expressway Server act as a gateway and convert the two protocol versions.

Endpoints register only with IPv6 addresses on an Expressway Server with IPv6 protocol enabled regardless of the state of IPv4 protocol.

Endpoints register with either IPv4 or IPv6 addresses, and endpoints decide which protocol to use by the format of the configured Expressway Server IP address.