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X.25 Type Numbers

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X.25 Type Numbers

X.25 Type Numbers


CCITT defines the high order two bits of the first octet of call user
data as follows:

00 - Used for other CCITT recomendations (such as X.29)
01 - Reserved for use by "national" administrative
10 - Reserved for use by international administrative authoorities
11 - Reserved for arbitrary use between consenting DTEs

Call User Data (hex)     Protocol                      Reference
-------------------      --------                      ---------

01                       PAD                               [GS2]
C5                       Blacker front-end descr dev       [AGM]
CC                       IP                         [RFC877,AGM]*
CD                       ISO-IP                            [AGM]             
CF                       PPP                           [RFC1598]
DD                       Network Monitoring                [AGM]

*NOTE: ISO SC6/WG2 approved assignment in ISO 9577 (January 1990).


[RFC877] Korb, J., "A Standard for the Transmission of IP Datagrams
         Over Public Data Networks", RFC 877, Purdue University,
         September 1983.

[RFC1598] Simpson, W., "PPPin X.25", RFC 1598, Daydreamer, March 1994.


[AGM]     Andy Malis <>

[GS2]     Greg Satz  <satz@CISCO.COM>


Connected: An Internet Encyclopedia
X.25 Type Numbers