Path: utzoo!mnetor!uunet!lll-winken!lll-tis!ames!hao!gatech!psuvax1!blitz!ehrlich From: ehrlich@blitz (Dan Ehrlich) Newsgroups: comp.protocols.tcp-ip Subject: Re: Link level Ethernet bridges (Modified)! does that mean joke? Message-ID: <3343@psuvax1.psu.edu> Date: 3 Mar 88 02:03:30 GMT References: <8802232312.AA02132@sneezy.lanl.gov> Sender: netnews@psuvax1.psu.edu Reply-To: ehrlich@blitz.cs.psu.edu (Dan Ehrlich) Organization: Department of Computer Science, Penn State University Lines: 59 Summary: It's my ball and I'll mak ethe rules! Once upon a time, in another job far, far, away I also encountered these mysterious aa:0:4:0:xx:xx addresses. Read on. Maybe someone can explain to me why DEC does what it does. :-) In article <8802232312.AA02132@sneezy.lanl.gov> cpw%sneezy@LANL.GOV (C. Philip Wood) writes: >... > > segment #1 segment #2 > > 8:0:20:1:33:1e Ha--| |--Hz 8:0:2b:3:92:23 > | | > 8:0:20:1:47:d2 Hb--| |--Hz-1 8:0:20:1:d7:e5 > |icmp icmp |--Hy 2:7:1:0:a6:59 > 8:0:9:0:65:3d Hp*--|echo-> echo->|--Hx aa:0:4:0:da:4 hummmm. ^^^^^^^^^^^^^ This mystery number is from a VAX running DECnet, either under VMS or Ultrix. It could also be from another vendor that has implemented DECnet's host->ethernet address mapping. DEC in their infinite wisdom uses one of the maintenance functions in the DE[QU]NA to change the physical ethernet address from what is in the ROM to this magic number. I do not know if they have gotten around to documenting how it is computed, but if memory serves me correctly it goes something like this: aa:0:4:0 is a magic constant that appears on any VAX running DECnet. da:4 is, get ready, the DECnet host and area number (byte swapped of course). Educated guess says that this is DECnet host 1.218. (Area number) * 1024 + (Host number). When I asked DEC why they do this I got numerous excuses. "Keeping a table mapping DECnet host numbers to Ethernet addresses would soak up too much memory" "We do not have anything like ARP and do not expect to in the near future" "Whoes name is on the fron of the 'Blue Book' anyway?". All of these are from the 1986 time frame and as I have not had reason to ask the question of late do know know if the responses are still valid. > nada <-reply | > . . > . . > . . > 8:0:20:1:66:6a Hq--| |--Hr 8:0:2b:3:91:5c > |----?[DEC lanBridge]?---| > > SUBNET 1 of Class B network 128.165, mask==0xfffffc00 > >There is probably alot of irreverent/irrelevant, but, it was fun. >... Dan Ehrlich The Pennsylvania State University, Department of Computer Science 333 Whitmore Laboratory, University Park, PA 16802 +1 814 863 1142 or +1 814 865 9723