[ih] X.25

[vinton cerf]

We used Internet to connect to x.25 networks. Univ Wisconsin was involved.
Larry Landerber had a lead role I believe

V

On Mon, Sep 29, 2025, 15:57 Jack Haverty via Internet-history <
internet-history at elists.isoc.org> wrote:

> Lots of info here:
>
> https://apps.dtic.mil/sti/tr/pdf/ADA137427.pdf   (See especially section
> 1.1.2)
>
> and here:
>
> https://apps.dtic.mil/sti/tr/pdf/ADA206354.pdf
>
> I don't recall if X.25 was ever used in the ARPANET, or in the public
> Internet as a way to interface to host computers.
>
> Sometime prior to 1983 we had added an X.25 interface to the core
> gateway at BBN, which enabled it to use the public X.25/X.75 network as
> an alternate path across the Atlantic to provide connectivity between
> the US and Europe.  There was also a gateway at UCL with an X.25
> interface.  As far as I remember there were never any host computers
> using X.25 to access those gateways, but the two gateways could interact
> over the X.25 world just as they did over other networks.
>
> The motivation for that so-called "VAN Gateway" was to allow projects in
> EU that were not ARPA-related to have connectivity to US sites on the
> ARPANET.  ARPA didn't want the SATNET resources (primarily the Intelsat
> IV-A satellite channel) to be used for non-ARPA projects.
>
> More info here: https://apps.dtic.mil/sti/tr/pdf/ADA122596.pdf (see page
> 35)
>
> At the time, the routing mechanisms of the Internet were unable to
> handle such "policy routing", but could only choose a route based on hop
> count.  IIRC, we had to use a few IP tricks to make the different types
> of traffic take the desired routes.  I think that was accomplished by
> giving some LANs in Europe two different network numbers.   To connect
> to a host on that LAN using the SATNET route, you would use an IP
> address on the "non-ARPA" network.  To connect to a host using the X.25
> route, you would use an IP address on the ARPA network.
>
> It was a kludge, but it worked.  It also put "policy routing" on the
> ICCB's to-do list for the next generation of routing protocols, and
> validated that X.25 could be used within the TCP/IP universe.
>
> Jack Haverty
>
>
>
>
> On 9/29/25 12:07, Michael Grant via Internet-history 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
>
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