The B4 component is used to connect the IPv4 customer network with the SP's IPv6 network. This function is typically performed at the customer's gateway device. The B4 component initiates a tunnel across the IPv6 network to the AFTR. The AFTR is the end point for the 4to6 tunnel and is the access point for the IPv4 portion of the network. After the tunnel is terminated, the AFTR perform Network Address Translation on the customer's IPv4 address to a public IPv4 and a dynamically assigned port number