RIP evolved as an industry standard and was popularized by its inclusion in the Berkeley Standard Distribution of UNIX (BSD UNIX). This first version of RIP (now sometimes called RIP-1 to differentiate it from later versions), was eventually standardized in RFC 1058. As part of this standard the original RIP-1 message format was defined, which of course serves RIP-1 itself, and is also the basis for the format used in later versions.

As explained in the general discussion on RIP operation, route information is exchanged in RIP through the sending of two different types of RIP messages: RIP Request and the RIP Response. These are transmitted as regular TCP/IP messages using the User Datagram Protocol (UDP), using UDP reserved port number 520. This port number is used as follows:

• RIP Request messages are sent to UDP destination port 520. They may have a source port of 520 or may use an ephemeral port number.
  
  
• RIP Response messages sent in reply to an RIP Request are sent with a source port of 520, and a destination port equal to whatever source port the RIP Request used.
  
  
• Unsolicited RIP Response messages (sent on a routine basis and not in response to a request) are sent with both the source and destination ports set to 520.