While OSPF was natively built to route IP and is itself a Layer 3 protocol that runs on top of IP, IS-IS is an OSI Layer 2 protocol. It is at the same layer as Connectionless Network Protocol (CLNP). The widespread adoption of IP may have contributed to OSPF's popularity. IS-IS does not use IP to carry routing information messages. OSPF version 2, on the other hand, was designed for IPv4. IS-IS is neutral regarding the type of network addresses for which it can route. This allowed IS-IS to be easily used to support IPv6. To operate with IPv6 networks, the OSPF protocol was rewritten in OSPF v3 (as specified in RFC 2740).