dennis@rlgvax.UUCP (Dennis Bednar) (01/13/86)
Could someone please explain what is meant by the
term "subnetting"?
--
Dennis Bednar Computer Consoles Inc. Reston VA 703-648-3300
{decvax,ihnp4,harpo,allegra}!seismo!rlgvax!dennis UUCPirwin@uiucdcs.CS.UIUC.EDU (01/17/86)
Laser Printer | Main Ethernet T-------O-------O-------O-------O-------O-------O-------T | | | | | Main Main Main Server Server Frame Frame Frame One Two Three | Subnet One T-------O-------O-------O-------O-------O-------T | | | | | Sun Sun Sun Sun Sun 170 170 170 120 120 #01 #02 #03 #01 #02 w/disk w/disk w/disk diskless diskless | Subnet Two T-------O-------O-------O-------O-------O-------T | | | | | Work Work Work Work Work Station Station Station Station Station In the above map: T is a terminator at coax end. O is the trancievers. | is the drop cables, transceiver to machine. Main Frame One is gateway to Subnet One. Sun 170 #01 is gateway to Subnet Two. Sun 120 #01 and #02 run diskless, using one of the 170s disks over ethernet. The advantage of the subnet, it keeps the diskless Suns traffic and work stations off of the main net, except when grabbing a file from a server or sending output to the printer, etc. Also, if one of the subnets fail, open coax, bad transceiver, etc., it will not hang the other nets at a different gateway level. All levels can access the printer, rlogins can get the work stations to main frame to check mail, etc. Note that the gateway machines require two ethernet interfaces each in the above illustration. Hope this helps. I think this is the application of the term as you are using it.
guy@sun.uucp (Guy Harris) (01/22/86)
> (discussion of a network with one "backbone" Ethernet and gateways on that > net to two subsidiary Ethernets) > ...I think this is the application of the term as you are using it. No, it isn't. He was most likely referring to the "Internet Standard Subnetting Procedure" described in RFC950. From that RFC: This memo discusses the utility of "subnets" of Internet networks, which are logically visible sub-sections of a single Internet network. For administrative or technical reasons, many organizations have chosen to divide one Internet network into several subnets, instead of acquiring a set of Internet network numbers. This memo specifies procedures for the use of subnets. These procedures are for hosts (e.g., workstations). The procedures used in and between subnet gateways are not fully described. Important motivation and background information for a subnetting standard is provided in RFC-940 [7]. ... 1. Motivation The original view of the Internet universe was a two-level hierarchy: the top level the Internet as a whole, and the level below it individual networks, each with its own network number. The Internet does not have a hierarchical topology, rather the interpretation of addresses is hierarchical. In this two-level model, each host sees its network as a single entity; that is, the network may be treated as a "black box" to which a set of hosts is connected. While this view has proved simple and powerful, a number of organizations have found it inadequate, and have added a third level to the interpretation of Internet addresses. In this view, a given Internet network is divided into a collection of subnets. The three-level model is useful in networks belonging to moderately large organizations (e.g., Universities or companies with more than one building), where it is often necessary to use more than one LAN cable to cover a "local area". Each LAN may then be treated as a subnet. There are several reasons why an organization might use more than one cable to cover a campus: - Different technologies: Especially in a research environment, there may be more than one kind of LAN in use; e.g., an organization may have some equipment that supports Ethernet, and some that supports a ring network. - Limits of technologies: Most LAN technologies impose limits, based on electrical parameters, on the number of hosts connected, and on the total length of the cable. It is easy to exceed these limits, especially those on cable length. - Network congestion: It is possible for a small subset of the hosts on a LAN to monopolize most of the bandwidth. A common solution to this problem is to divide the hosts into cliques of high mutual communication, and put these cliques on separate cables. - Point-to-Point links: Sometimes a "local area", such as a university campus, is split into two locations too far apart to connect using the preferred LAN technology. In this case, high-speed point-to-point links might connect several LANs. An organization that has been forced to use more than one LAN has three choices for assigning Internet addresses: 1. Acquire a distinct Internet network number for each cable; subnets are not used at all. 2. Use a single network number for the entire organization, but assign host numbers without regard to which LAN a host is on ("transparent subnets"). 3. Use a single network number, and partition the host address space by assigning subnet numbers to the LANs ("explicit subnets"). Each of these approaches has disadvantages. The first, although not requiring any new or modified protocols, results in an explosion in the size of Internet routing tables. Information about the internal details of local connectivity is propagated everywhere, although it is of little or no use outside the local organization. Especially as some current gateway implementations do not have much space for routing tables, it would be good to avoid this problem. ... Your example is an instance of that general kind of subnetting, but "TCP/IP Subnetting" normally refers to the method of address assignment described in "3" above, which is the official way of doing IP subnetting. I am posting the text of RFC950 "Internet Standard Subnetting Procedure", RFC940 "Toward an Internet Standard Scheme for Subnetting", and RFC917 "Internet Subnets" to net.dcom. Guy Harris
ron@brl-smoke.ARPA (Ron Natalie <ron>) (01/28/86)
> > [CUTE PICTURE ENCLOSED] > > The advantage of the subnet, it keeps the diskless Suns traffic > and work stations off of the main net, except when grabbing a file > from a server or sending output to the printer, etc. > > Also, if one of the subnets fail, open coax, bad transceiver, etc., > it will not hang the other nets at a different gateway level. All levels > can access the printer, rlogins can get the work stations to main frame > to check mail, etc. > > Note that the gateway machines require two ethernet interfaces each > in the above illustration. > > Hope this helps. I think this is the application of the term as you > are using it. Actually, none of this are points for subnetting. What he is showing is why you don't want your entire plant to be one net. IP subnetting is simply the practice of using a single IP network number across multiple physical nets. The advantage is that outside your organization, noone need know about the topology of your local configuration, they see a single IP netork number referencing an apparent single network. The trick is that within one subnet has to realize that some of the hosts with the same network number are on different nets. Essentially, the idea is that people rarely have more than 256 hosts supported by single class C net number, but frequently have multiple nets. There is no reason to burden the rest of the internet system with a mutliple net numbers that each refer to a handful of hosts in the same system when you can do the work locally and divide up a single network nubmer. A plan for subnetting is available as an RFC from the NIC. The 4.3 BSD release will support this. -Ron