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Since VSLM is supported only by classless routing protocols, if VSLM was introduced in 1987(RFC 1009) and all classless protocol(RIPv2 developed in 1993 , OSPF standardized in 1989 RFC 1131 ) cited by my Computer Networks book (Tanenbaum 5th edition) are posterior to VSLM introduction, what were the classless routing protocols used with VLSM when it was first introduced?

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Actually, VLSM is first mentioned in RFC 950 from 1985. It enabled subnetting the classful address ranges, so that you could actually split a classful range into a more complex network. That eventually evolved to CIDR and obsoleted classful networking.

The first routing protocol actually supporting VLSM seems to be OSPF v1 from RFC 1131 in 1989. Likely, most people used static routing back then - as far as I know static routes have always supported arbitrary subnet prefix lengths (or even non-contiguous masks).

Remember that back in 1985, the total number of hosts on the Internet was around 2,000. Any network with more than a dozen hosts would have been considered huge.

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  • Don't forget EGP (1984) and BGP (1989) – Ron Trunk Jun 16 at 18:42
  • EGP only seemed to support classes in 1984 (RFC 950), same with BGP (RFC 1105) - neither has provisions to specify a prefix length. – Zac67 Jun 16 at 18:52
  • In a pure classful network, netmasks are not specified -- they are determined by the address. Thus until VLSM came about subnetting was not possible. It a common error because so few people today have ever encountered anything that didn't support VLSM. – Ricky Jun 17 at 0:17
  • @RickyBeam "Thus until VLSM came about subnetting was not possible." According to what I have studied and understood subnetting was possible also before VLSM but there was the constraint (due to classful addressing) to divide an IP network in a fixed number of fixed sized subnets with each subnet having the subnet mask of the same lenght – Giuseppe Jun 17 at 9:18
  • VLSM was the first step to actually announce variable-length routes to other routers. Before that, static routing was the only choice. CIDR then actively obsoleted classful routing, so the old protocols had to be updated (or go). – Zac67 Jun 17 at 10:43

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