per@erix.UUCP (Per Hedeland) (05/05/88)
Excuse me if the question below is trivial, but I really haven't seen much
discussion on subnetting, and neither RFC reading nor local asking-around
has gotten me very far...
This is the scenario: The basic structure of our LAN is a backbone segment,
to which a number of Sun server/client groups are connected. Each client group
has it's own Ether segment, with the server acting as gateway to the backbone.
Typically there are 5-10 clients in each group. There are currently some 20
such groups, but predictions are for hundreds in the not too distant future,
i.e. considering other connected equipment, far more than 256 addresses are
required for the backbone.
Due to us not being connected to the Internet, and inadequate planning of the
growth of the LAN, network numbering is a mess, which we would like to clean up
as soon as possible, and this prompts my question: It seems to be a terrible
waste of adress space to use a separate class C number for each of the client
groups, so we figured that subnetting would be appropriate, but how do we use
it to an advantage?
Specifically, given the abovementioned structure, there's a conflict between:
a) It appears that the intended use of subnetting assumes that all the "subs"
of a "whole" net are interconnected, i.e. the backbone and the client
segments in our case should be "subs" of the same "whole" - in particular,
"automatic" routing by means of routed (which we desire) will not work
otherwise, as far as I can understand.
and:
b) "Subs" of a given "whole" must be of equal size.
We do believe that the arguments for the current structure (such as handling
the load from diskless clients, avoiding extra cabling given the physical
location of servers and clients, allowing use of the backbone for e.g. DECnet
traffic) are valid, and it seems rather silly having to modify it to
accommodate the IP adressing scheme. But of course, neither the structure nor
the requirement for "automatic" routing are carved in stone, i.e. any and all
suggestions are welcome (preferrably via e-mail, and I will summarize to the
net if requested).
Thanks In Advance
--Per Hedeland
Internet: per@erix.ericsson.se
Non-MX: per%erix.ericsson.se@uunet.uu.net
UUCP: {mcvax,munnari,uunet}!enea!erix!perbc@halley.UUCP (Bill Crews) (05/06/88)
In article <1607@erix.UUCP> per@erix.ericsson.se (Per Hedeland) writes: > >Typically there are 5-10 clients in each group. There are currently some 20 >such groups, but predictions are for hundreds in the not too distant future, >i.e. considering other connected equipment, far more than 256 addresses are >required for the backbone. > It seems to be a terrible >waste of adress space to use a separate class C number for each of the client >groups, so we figured that subnetting would be appropriate, It does to me, too. So, why not just use class B addresses? Based on the criteria you dictate, that would seem to be adequate for now and the future. -bc -- Bill Crews Tandem Computers bc@halley.UUCP Austin, Texas ..!rutgers!im4u!halley!bc (512) 244-8350
joe@tekbspa.UUCP (Joe Angelo) (05/09/88)
in article <358@halley.UUCP>, bc@halley.UUCP (Bill Crews) says: > Xref: tekbspa comp.dcom.lans:150 comp.protocols.tcp-ip:495 > > > It does to me, too. So, why not just use class B addresses? Based on the > criteria you dictate, that would seem to be adequate for now and the future. > Now, the classes of IP addresses is simple enough to understand -- But what about subnetting? When does one want to *really* use two IP network address on the same cable? And what performance advantages does this give you? Were does the netmask come it at? Is subnetting just a nice admistrativia thing? Or does your local enet board not receive the packets, period? Or is it the high level software that ignores the packet? Does anything really ignore anything? -- "I'm trying Joe Angelo -- Senior Systems Engineer/Systems Manager to think at Teknekron Software Systems, Palo Alto 415-325-1025 but nothing happens!" uunet!tekbspa!joe -OR- tekbspa!joe@uunet.uu.net
kwe@bu-cs.BU.EDU (kwe@bu-it.bu.edu (Kent W. England)) (05/09/88)
In article <203@tekbspa.UUCP> joe@tekbspa.UUCP (Joe Angelo) writes: > >But what about subnetting? ... >Is subnetting just a nice admistrativia thing? Subnetting is not at all administrivia if you are trying to plug two autonomous systems together. An autonomous system is just an administrative boundary between networks. You may have many ASs within your company or campus or the chief boundary may be between your Network and the Internet. If you have ever tried just plugging some other ASs router onto your homogeneous Ethernet, you would have a feel for the grief that you and the other system admin are in for. You don't have to hide your own admin trivia behind one Class A/B network address, but it sure makes life easier. It might be smart to prepare for installing routers by subnetting addresses while still interconnecting segments with repeaters and/or bridges. You can set up a Class B with pseudo-subnetting and, at some convenient point, replace repeaters and bridges with real IP routers without having to change addresses. (Hosts that understand subnets must be properly instructed about the true network architecture. Hosts that don't understand subnets must always be fooled. :-) >Or does your local >enet board not receive the packets, period? Or is it the high >level software that ignores the packet? Does anything really ignore >anything? > If your network is not really subnetted (just looks that way), then broadcasts are still heard everywhere, your net will still meltdown, your local enet boards will still process tens of broadcasts per second. :-) Just setting up addresses to look subnetted is not subnetting, not until you actually have subnet routing. That's my view... Kent England, Boston University
ed@mtxinu.UUCP (Ed Gould) (05/10/88)
>Now, the classes of IP addresses is simple enough to understand -- > >But what about subnetting? When does one want to *really* use >two IP network address on the same cable? And what performance >advantages does this give you? Were does the netmask come it at? The purpose of subnetting is not to run multiple IP addresses on the same cable, but to make a local collection of networks appear as if it were one network from the outside. To do this, one takes a standard IP adress (ususally a Class B address, but this isn't required) and uses some of the bits that are normally the "host number" part of the address as if they were part of the "network number." The netmask determines the division between host part and network part that is used locally. For example, consider the following /etc/hosts excerpt, which lists several Class B addresses. Keep in mind a netmask of 0xFFFFFF00 (the normal Class B netmask is 0xFFFF0000). 129.1.1.1 main-sys 129.1.2.1 main-sys-gw 129.1.2.2 second-sys 129.1.2.3 third-sys One topology this could represent is net to outside internal net (129.1.1) (129.1.2) ________ _________________________________ | | | | | | | | ----------- ------------- -------------- | | | | | | | main-sys| | second-sys| | third-sys | | | | | | | ----------- ------------- -------------- In this case, there are two physical networks: one connecting the three machines locally, and one connecting the main system to the outside world. To the outside world, the three machines look as if they are connected together on a single Class B network. Internally, though, they look as if they were on two separate Class C networks with a gateway. -- Ed Gould mt Xinu, 2560 Ninth St., Berkeley, CA 94710 USA {ucbvax,uunet}!mtxinu!ed +1 415 644 0146 "I'll fight them as a woman, not a lady. I'll fight them as an engineer."
per@erix.UUCP (Per Hedeland) (06/01/88)
While it may be a bit late, I thought I should post a short summary of the information I got in response to my query a few weeks back - it did generate some discussion, after all. If you recall, the issue was how to use subnetting on a network structure with a big backbone net and many small ones hanging off of it. I think the below is the essence of what I found out; I also have a pile of mail for those particularly interested. - Unequal-sized subnets, whether or not a Good Thing, is not currently implemented in any generally accepted way (and thus of little interest to us). - Partitioned subnets (i.e. subnets of a given net interconnected only by some other net) are out. - One can have several logical subnets on the same wire (i.e. the backbone in our case), thus effectively increasing the address space, at the expense of some manual "routing" (i.e. gateways have to be explicitly told that the different subnets can be accessed via the same interface - hosts on such a wire can be "fooled" into believing that it's all one (bigger) subnet). We will probably go for the latter method, with a long-term goal of splitting our backbone by using dedicated routers. Thanks to all who offered help and advice! ---Per Hedeland