Port 321: Pip — The Road Not Taken

Port 321 belongs to the well-known ports range (0-1023), the tier of ports assigned by IANA through formal review processes. But unlike most well-known ports that carry daily traffic across the Internet, port 321 is assigned to a protocol that never shipped: Pip, the P Internet Protocol.

What Pip Was

Between 1992 and 1993, engineers at Bellcore (now Telcordia Technologies) developed Pip as a candidate replacement for IPv4.¹ The Internet was growing faster than anyone expected, and the cracks in IPv4's addressing system were starting to show. Pip was designed as a general-purpose Internet protocol that could evolve to meet all foreseeable requirements—flexible addressing, scalable routing, provider-independent addressing.²

Paul Francis led the effort. The architecture was documented in RFC 1621 in May 1994.³ Pip was assigned IP version number 8 and TCP port 321. Everything was ready.

What Actually Happened

In mid-1993, Pip was merged with another IPv4 replacement candidate called the Simple Internet Protocol (SIP), creating SIPP (SIP Plus).⁴ SIPP continued to evolve through the IETF's IPng (IP Next Generation) effort and eventually became what we now know as IPv6.⁵

Pip itself never carried a single packet across the production Internet.

RFC 1621 now includes a note in its preamble: "Any text that indicates that Pip is an intended replacement for IP should be ignored." The document serves as a historical record of ideas developed during the Pip project, not a specification for a deployed protocol.¹

Why This Port Still Exists

Port 321 remains assigned to Pip in IANA's registry, registered to Gordon Mohr.⁶ It's a monument to a protocol that almost was—a permanent marker for a road the Internet chose not to take.

The Internet Protocol we use today (IPv6) carries DNA from Pip, SIP, and other candidates from that era. Ideas from Pip's flexible addressing and routing architecture influenced the eventual design. Port 321 is a reminder that even rejected proposals leave their mark.

Checking This Port

To see if anything is listening on port 321:

Linux/Mac:

sudo lsof -i :321
netstat -an | grep 321

Windows:

netstat -an | findstr :321

You'll almost certainly find nothing. Port 321 is quiet—waiting for a protocol that will never arrive.

What This Port Means

Well-known ports are supposed to represent services that matter to the Internet's infrastructure. Port 321 matters differently. It represents the ideas that didn't win but shaped what did. Every technology decision is a fork in the road. Pip was the path not taken, but its port number remains in the registry, a permanent reminder that the Internet we have is only one of many possible Internets.

Port 321 is a monument to engineering that never shipped but influenced what did. That's worth remembering.