IP Sub-Networking Mini-Howto
  Robert Hart, hartr@interweft.com.au
  v1.1, 30 August 2001

  This document describes why and how to subnetwork an IP network - that
  is using a single A, B or C Class network number to function correctly
  on several interconnected networks.

  1.  Copyright

  This document is distributed under the terms of the GNU Public License

  This document is directly supported by InterWeft IT Consultants
  (Melbourne, Australia).

  The latest version of this document is available at the InterWeft WWW
  site at InterWeft IT Consultants <http://www.interweft.com.au/> and
  from The Linux Documentation Project <http://sunsite.unc.edu/LDP>.

  2.  Introduction

  With available IP network numbers rapidly becoming an endangered
  species, efficient use of this increasingly scarce resource is

  This document describes how to split a single IP network number up so
  that it can be used on several different networks.

  This document concentrates on C Class IP network numbers - but the
  principles apply to A and B class networks as well.

  2.1.  Other sources of information

  There are a number of other sources of information that are of
  relevance for both detailed and background information on IP numbers.
  Those recommended by the author are:-

  ·  The Linux Network Administrators Guide

  ·  The Linux System Administration Guide

  ·  TCP/IP Network Administration by Craig Hunt, published by O'Reilly
     and Associates <http://www.ora.com/catalog/tcp/noframes.html>.

  3.  The Anatomy of IP numbers

  Before diving into the delight of sub-networking, we need to establish
  some IP number basics.

  3.1.  IP numbers belong to Interfaces - NOT  hosts!

  First of all, let's clear up a basic cause of misunderstanding - IP
  numbers are not assigned to hosts. IP numbers are assigned to network
  interfaces on hosts.
  Eh - what's that?

  Whilst many (if not most) computers on an IP network will possess a
  single network interface (and have a single IP number as a
  consequence), this is not the only way things happen. Computers and
  other devices can have several (if not many) network interfaces - and
  each interface has its own IP number.

  So a device with 6 active interfaces (such as a router) will have 6 IP
  numbers - one for each interface to each network to which it is
  connected. The reason for this becomes clear when we look at an IP

  Despite this, most people refer to host addresses when referring to an
  IP number. Just remember, this is simply shorthand for the IP number
  of this particular interface on this host. Many (if not the majority)
  of devices on the Internet have only a single interface and thus a
  single IP number.

  3.2.  IP Numbers as "Dotted Quads"

  In the current (IPv4) implementation of IP numbers, IP numbers consist
  of 4 (8 bit) bytes - giving a total of 32 bits of available
  information.  This results in numbers that are rather large (even when
  written in decimal notation). So for readability (and organisational
  reasons) IP numbers are usually written in the 'dotted quad' format.
  The IP number


  is an example of this - 4 (decimal) numbers separated by (.) dots.

  As each one of the four numbers is the decimal representation of an 8
  bit byte, each of the 4 numbers can range from 0 to 255 (that is take
  on 256 unique values - remember, zero is a value too).

  In addition, part of the IP number of a host identifies the network on
  which the host resides, the remaining 'bits' of the IP number identify
  the host (oops - network interface) itself. Exactly how many bits are
  used by the network ID and how many are available to identify hosts
  (interfaces) on that network is determined by the network 'class'.

  3.3.  Classes of IP Networks

  There are three classes of IP numbers

  ·  Class A IP network numbers use the leftmost 8 bits (the leftmost of
     the dotted quads) to identify the network, leaving 24 bits (the
     remaining three dotted quads) to identify host interfaces on that
     Class A addresses always have the leftmost bit of the leftmost byte
     a zero - that is a decimal value of 0 to 127 for the first dotted
     quad. So there are a maximum of 128 class A network numbers
     available, with each one containing up to 33,554,430 possible

     However, the networks (known as the default route) and (the loop back network) have special meanings and are not
     available for use to identify networks. So there are only 126
     available A class network numbers.

  ·  Class B IP network numbers use the leftmost 16 bits (the leftmost
     two dotted quads) to identify the network, leaving 16 bits (the
     last two dotted quads) to identify host interfaces. Class B
     addresses always have the leftmost 2 bits of the leftmost byte set
     to 1 0. This leaves 14 bits left to specify the network address
     giving 32767 available B class networks. B Class networks thus have
     a range of 128 to 191 for the first of the dotted quads, with each
     network containing up to 32,766 possible interfaces.

  ·  Class C IP network numbers use the leftmost 24 bits (the leftmost
     three bytes) to identify the network, leaving 8 bits (the rightmost
     byte) to identify host interfaces. Class C addresses always start
     with the leftmost 3 bits set to 1 1 0 or a range of 192 to 255 for
     the leftmost dotted quad. There are thus 4,194,303 available C
     class network numbers, each containing 254 interfaces. (C Class
     networks with the first byte greater than 223 are however reserved
     and unavailable for use).

  In summary:

               Network class   Usable range of first byte values (decimal)
                       A                 1 to 126
                       B               128 to 191
                       C               192 to 254

  There are also special addresses that are reserved for 'unconnected'
  networks - that is networks that use IP but are not connected to the
  Internet, These addresses are:-

  ·  One A Class Network

  ·  16 B Class Networks -

  ·  256 C Class Networks -

  You will note that this document uses these sequences throughout to
  avoid confusion with 'real' networks and hosts.

  3.4.  Network numbers, interface addresses and broadcast addresses

  IP numbers can have three possible meanings:-

  ·  the address of an IP network (a group of IP devices sharing common
     access to a transmission medium - such as all being on the same
     Ethernet segment). A network number will always have the interface
     (host) bits of the address space set to 0 (unless the network is
     sub-networked - as we shall see);

  ·  the broadcast address of an IP network (the address used to 'talk',
     simultaneously, to all devices in an IP network). Broadcast
     addresses for a network always have the interface (host) bits of
     the the address space set to 1 (unless the network is sub-networked
     - again, as we shall see).

  ·  the address of an interface (such as an Ethernet card or PPP
     interface on a host, router, print server etc). These addresses can
     have any value in the host bits except all zero or all 1 - because
     with the host bits all 0, the address is a network address and with
     the host bits all 1 the address is the broadcast address.

  In summary and to clarify things

       For an A class network...
       (one byte of network address space followed by three bytes of host
       address space)

      is an A Class  network number  because all the host
                       bits of the address space are 0
      is a host address on this network
      is the broadcast address of this network
                       because all the host bits of the address space are 1

       For a B class network...
       (two bytes of network address space followed by two bytes of host
       address space)

      is a B Class network number
      is a host address on this network
      is the network broadcast address

       For a C Class network...
       (three bytes of network address space followed by one byte of host
       address space)

      is a C Class network number
      is a host address on this network
      is the network broadcast address

  Almost all IP network numbers remaining available for allocation at
  present are C Class addresses.

  3.5.  The network mask

  The network mask is more properly called the subnetwork mask. However,
  it is generally referred to as the network mask.

  It is the network mask and its implications on how IP addresses are
  interpreted locally on an IP network segment that concerns us most
  here, as this determines what (if any) sub-networking occurs.

  The standard (sub-) network mask is all the network bits in an address
  set to '1' and all the host bits set to '0'. This means that the
  standard network masks for the three classes of networks are:-

  ·  A Class network mask:

  ·  B Class network mask:

  ·  C Class network mask:

  There are two important things to remember about the network mask:-

  ·  The network mask affects only the local interpretation of local IP
     numbers (where local means on this particular network segment);

  ·  The network mask is not an IP number - it is used to modify how
     local IP numbers are interpreted locally.

  4.  What are subnets?

  A subnet is a way of taking a single IP network address and locally
  splitting it up so that this single network IP address can actually be
  used on several interconnected local networks. Remember, a single IP
  network number can only be used on a single network.

  The important word here is locally: as far as the world outside the
  machines and physical networks covered by the sub-netted IP network
  are concerned, nothing whatsoever has changed - it is still just a
  single IP network. This is important - sub-networking is a local
  configuration and is invisible to the rest of the world.

  5.  Why subnetwork?

  The reasons behind sub-networking date back to the early specification
  of IP - where just a few sites were running on Class A network
  numbers, which allow for millions of connected hosts.

  It is obviously a huge traffic and administration problem if all IP
  computers at a large site need to be connected to the same network:
  trying to manage such a huge beast would be a nightmare and the
  network would (almost certainly) collapse under the load of its own
  traffic (saturate).

  Enter sub-networking: the A class IP network address can be split up
  to allow its distribution across several (if not many) separate
  networks.  The management of each separate network can easily be
  delegated as well.

  This allows small, manageable networks to be established - quite
  possibly using different networking technologies. Remember, you cannot
  mix Ethernet, Token Ring, FDDI, ATM etc on the same physical network -
  they can be interconnected, however!

  Other reasons for sub-networking are:-

  ·  Physical site layout can create restrictions (cable run lengths) in
     terms of the how the physical infrastructure can be connected,
     requiring multiple networks. Sub-networking allows this to be done
     in an IP environment using a single IP network number.
     This is in fact now very commonly done by ISPs who wish to give
     their permanently connected clients with local networks static IP

  ·  Network traffic is sufficiently high to be causing significant slow
     downs. By splitting the network up using subnetworks, traffic that
     is local to a network segment can be kept local - reducing overall
     traffic and speeding up network connectivity without requiring more
     actual network bandwidth;

  ·  Security requirements may well dictate that different classes of
     users do not share the same network - as traffic on a network can
     always be intercepted by a knowledgeable user. Sub-networking
     provides a way to keep the marketing department from snooping on
     the R & D network traffic (or students from snooping on the
     administration network)!

  ·  You have equipment which uses incompatible networking technologies
     and need to interconnect them (as mentioned above).

  6.  How to subnetwork a IP network number

  Having decided that you need to subnetwork your IP network number, how
  do you go about it? The following is an overview of the steps which
  will then be explained in detail:-

  ·  Set up the physical connectivity (network wiring and network
     interconnections - such as routers;

  ·  Decide how big/small each subnetwork needs to be in terms of the
     number of devices that will connect to it - ie how many usable IP
     numbers are required for each individual segment.

  ·  Calculate the appropriate network mask and network addresses;

  ·  Give each interface on each network its own IP address and the
     appropriate network mask;

  ·  Set up the routes on the routers and the appropriate gateways,
     routes and/or default routes on the networked devices;

  ·  Test the system, fix problems and then relax!

  For the purpose of this example, we will assume we are sub-networking
  a single C class network number:

  This provides for a maximum of 254 connected interfaces (hosts), plus
  the obligatory network number ( and broadcast address

  6.1.  Setting up the physical connectivity

  You will need to install the correct cabling infrastructure for all
  the devices you wish to interconnect designed to meet your physical

  You will also need a mechanism to interconnect the various segments
  together (routers, media converters etc.).

  A detailed discussion of this is obviously impossible here. Should you
  need help, there are network design/installation consultants around
  who provide this sort of service. Free advice is also available on a
  number of Usenet news groups (such as comp.os.linux.networking).

  6.2.  Subnetwork sizing

  There is a play off between the number of subnetworks you create and
  'wasted' IP numbers.

  Every individual IP network has two addresses unusable as interface
  (host) addresses - the network IP number itself and the broadcast
  address. When you subnetwork, each subnetwork requires its own, unique
  IP network number and broadcast address - and these have to be valid
  addresses from within the range provided by the IP network that you
  are sub-networking.

  So, by sub-networking an IP network into two separate subnetworks,
  there are now two network addresses and two broadcast addresses -
  increasing the 'unusable' interface (host) addresses; creating 4
  subnetworks creates eight unusable interface (host) addresses and so

  In fact the smallest usable subnetwork consists of 4 IP numbers:-

  ·  Two usable IP interface numbers - one for the router interface on
     that network and one for the single host on that network.

  ·  One network number.

  ·  One broadcast address.

  Quite why one would want to create such a small network is another
  question! With only a single host on the network, any network
  communication must go out to another network. However, the example
  does serve to show the law of diminishing returns that applies to sub-

  In principle, you can only divide your IP network number into 2^n
  (where n is one less that the number of host bits in your IP network
  number) equally sized subnetworks (you can subnetwork a subnetwork and
  combine subnetworks however).

  So be realistic about designing your network design - you want the
  minimum number of separate local networks that is consistent with
  management, physical, equipment and security constraints!

  6.3.  Calculating the subnetwork mask and network numbers

  The network mask is what performs all the local magic of dividing an
  IP network into subnetworks.

  The network mask for an un-sub-networked IP network number is simply a
  dotted quad which has all the 'network bits' of the network number set
  to '1' and all the host bits set to '0'.

  So, for the three classes of IP networks, the standard network masks

  ·  Class A (8 network bits) :

  ·  Class B (16 network bits):

  ·  Class C (24 network bits):

  The way sub-networking operates is to borrow one or more of the
  available host bits and make then make interfaces locally interpret
  these borrowed bits as part of the network bits. So to divide a
  network number into two subnetworks, we would borrow one host bit by
  setting the appropriate bit in the network mask of the first (normal)
  host bit to '1'.

  For a C Class address, this would result in a netmask of

  For our C Class network number of, these are some of the
  sub-networking options you have:-

  No of      No of
  subnets    Hosts/net    netmask
  2            126 (11111111.11111111.11111111.10000000)
  4             62 (11111111.11111111.11111111.11000000)
  8             30 (11111111.11111111.11111111.11100000)
  16            14 (11111111.11111111.11111111.11110000)
  32             6 (11111111.11111111.11111111.11111000)
  64             2 (11111111.11111111.11111111.11111100)

  In principle, there is absolutely no reason to follow the above way of
  subnetworking where network mask bits are added from the most
  significant host bit to the least significant host bit. However, if
  you do not do it this way, the resulting IP numbers will be in a very
  odd sequence! This makes it extremely difficult for us humans to
  decide to which subnetwork an IP number belongs as we are not too good
  at thinking in binary (computers on the other hand are and will use
  whatever scheme you tell them with equal equanimity).

  Having decided on the appropriate netmask, you then need to work out
  what the various Network and broadcast addresses are - and the IP
  number range for each of these networks. Again, considering only a C
  Class IP Network number and listing only the final (host part) we

  Netmask         Subnets Network B'cast  MinIP   MaxIP   Hosts  Total Hosts
      128            2       0     127       1     126    126
                           128     255     129     254    126     252

      192            4       0      63       1      62     62
                            64     127      65     126     62
                           128     191     129     190     62
                           192     255     193     254     62     248

      224            8       0      31       1      30     30
                            32      63      33      62     30
                            64      95      65      94     30
                            96     127      97     126     30
                           128     159     129     158     30
                           160     191     161     190     30
                           192     223     193     222     30
                           224     255     225     254     30     240

  As can be seen, there is a very definite sequence to these numbers,
  which make them fairly easy to check. The 'downside' of sub-networking
  is also visible in terms of the reducing total number of available
  host addresses as the number of subnetworks increases.

  With this information, you are now in a position to assign host and
  network IP numbers and netmasks.

  7.  Routing

  If you are using a Linux PC with two network interfaces to route
  between two (or more) subnets, you need to have IP Forwarding enabled
  in your kernel. Do a

          cat /proc/ksyms | grep ip_forward

  You should get back something like...

  00141364 ip_forward_Rf71ac834

  If you do not, then you do not have IP-Forwarding enabled in your
  kernel and you need to recompile and install a new kernel.

  For the sake of this example, let us assume that you have decided to
  subnetwork you C class IP network number into 4 subnets
  (each of 62 usable interface/host IP numbers). However, two of these
  subnets are being combined into a larger single network, giving three
  physical networks.
  These are :-

  Network         Broadcast       Netmask                 Hosts         62         62         124 (see note)

  Note: the reason the last network has only 124 usable network
  addresses (not 126 as would be expected from the network mask) is that
  it is really a 'super net' of two subnetworks. Hosts on the other two
  networks will interpret as the network address of the
  'non-existent' subnetwork. Similarly, they will interpret as the broadcast address of the 'non-existent'

  So, if you use or 192 as host addresses on the third
  network, then machines on the two smaller networks will not be able to
  communicate with them.

  This illustrates an important point with subnetworks - the usable
  addresses are determined by the SMALLEST subnetwork in that address

  7.1.  The routing tables

  Let us assume that a computer running Linux is acting as a router for
  this network. It will have three network interfaces to the local LANs
  and possibly a fourth interface to the Internet (which would be its
  default route.

  Let us assume that the Linux computer uses the lowest available IP
  address in each subnetwork on its interface to that network. It would
  configure its network interfaces as

  Interface       IP Address      Netmask

  The routing it would establish would be

  Destination     Gateway         Genmask         Iface eth0 eth1 eth2

  On each of the subnetworks, the hosts would be configured with their
  own IP number and net mask (appropriate for the particular network).
  Each host would declare the Linux PC as its gateway/router, specifying
  the Linux PCs IP address for its interface on to that particular

  Robert Hart Melbourne, Australia March 1997.
howtos/ip-subnetworking.txt (192860 views) · Last modified: 2005/09/14 01:23 (external edit)
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