nyway. Some protocol architectures are also in general adapted well for proxying - for instance Internet mail [45]. A proxy is, simplifying heavily, an entity that acts like a server to the client, and a client towards the upstream server. For instance, a web proxy takes HTTP requests from the client side and sends the request to the server, which is supposed to provide the actual service. As there are two separate connections, the connection between the client and the proxy, and the proxy and the server, these can also use different IP versions. For instance, a client can be in an IPv6 network and use a proxy to access a service in an IPv4 network as long as the proxy has access to both networks. Internet mail was used as an example earlier for a protocol that is easily proxied. This is due to the email protocol architecture. The email clients connect to servers for receiving and sending email. The servers are then connected to other servers to deliver the mail to the server, which serves the email recipient. Thus, as in web proxies, the connection between the client and the server is completely separate from the connections between the servers. Again, the email servers can provide the service to their clients on an IP version independent of the connections between the servers themselves. Address and Port Sharing Address and port sharing, often referred to as A+P or port-restricted IPv4 addressing, was originally described in RFC 6346 [20]. Section 5.3.6 has already described the basics of address sharing using port ranges, and a MAP based solution approach. Concatenating an IPv4 address and a transport protocol (e.g. Transport Control Protocol (TCP) or User Datagram Protocol (UDP)) port number effectively gives 48 bits of total 'IPv4' address Deploying IPv6 in 3GPP Networks space. Of course specialized A+P aware routing functions have to be deployed in the access network to make address sharing happen. This section will not repeat the content of Section 5.3.6, but there have been multiple other proposals that a