Which protocol supports CIDR?

BGP4, or Border Gateway Protocol version 4, is the correct answer because it was specifically designed to support Classless Inter-Domain Routing (CIDR). CIDR is a method for allocating IP addresses and routing Internet Protocol packets, which allows for more efficient use of the available IP address space by enabling more flexible subnetting and aggregation of network prefixes.

BGP4 handles the advertising of CIDR blocks (the prefixes) rather than fixed classful network addresses, which allows for a more scalable and manageable routing system, particularly on the internet. This practice helps in minimizing the number of routes that need to be maintained in the routing tables by enabling the summarization of multiple smaller prefixes into a single larger one.

The other protocols mentioned do not have the same level of support for CIDR. For example, while RIP version 2 can support more flexible networks compared to its predecessor, it is still limited by certain classful network characteristics and is not as robust in managing CIDR as BGP4. Likewise, OSPF and EIGRP are more focused on intra-domain routing and may have different ways of handling routing information but primarily operate within a single administrative domain rather than having the same level of attention to IPv4 address aggregation and CIDR