[ih] X.25

[John Day]

X.25 was never used between IMPs.

The X.25 spec was very specific: It was a DCE to DTE protocol. ;-) IOW, Host to Router, or in this case Host to IMP.  X.25 could be used to replace BBN 1822.
(DCE = Data Communicating Equipment; DTE = Data Terminating Equipment.) Don’t you love CCITT terminology!!  ;-)



> On Sep 29, 2025, at 15:07, Michael Grant via Internet-history <internet-history at elists.isoc.org> wrote:
> 
> X.25 has been mentioned a few times on this list in the context some IMPs could talk to other IMPs over X.25 and somehow parts of the net (arpanet?) were connected over X.25.
> 
> X.25 addresses were sort of like phone numbers, they don't map on to IP addresses (or IMP/HOST).  And data was charged per packet, per byte, and iirc, per connection time.  And it was connection oriented though there was a sort of datagram mode called "fast connect".
> 
> How was it used in the early Internet?  Was there some static file passed around with how to route to something and which X.25 address to connect to?  Or was each "link" to other IMPs treated as a point to point connection with some configured X.25 address?
> 
> When I was working with the OSI protocols, X.25 was somehow supposed to be used as a network layer but I never could imagine how that was supposed to work in any practical way.  CLNP has a really long address that you could embed an X.25 address in but it seemed senseless (if you used CLNP). Who would be paying for that if something got routed over your link!  It would cost you a fortune!
> 
> So I'm curious, how did X.25 fit into things in the early internet?  Was it used much?  I just never could understand how X.25 was anything like the Internet in the OSI world if you had an X.25 connection.
> 
> Michael Grant
> -- 
> Internet-history mailing list
> Internet-history at elists.isoc.org
> https://elists.isoc.org/mailman/listinfo/internet-history
> -
> Unsubscribe: https://app.smartsheet.com/b/form/9b6ef0621638436ab0a9b23cb0668b0b?The%20list%20to%20be%20unsubscribed%20from=Internet-history