-
-
-
-IP Sub-Networking Mini-Howto
-
-<author>Robert Hart, <tt/hartr@interweft.com.au/
-<date>v1.1, 30 August 2001
-
-<!-- correct typos reported by users, 30 August 2001 -->
-<!-- Greg Ferguson / linux-howto@metalab.unc.edu -->
-
-<abstract>
-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. </abstract>
-
-<!-- Copyright 1997, Robert Hart -->
-
-<sect>Copyright
-<p>
-This document is distributed under the terms of the GNU Public License (GPL).
-
-<p>
-This document is directly supported by InterWeft IT Consultants
-(Melbourne, Australia).
-
-<p>
-The latest version of this document is available at the InterWeft WWW
-site at <url url="http://www.interweft.com.au/" name="InterWeft IT
-Consultants"> and from <url url="http://sunsite.unc.edu/LDP" name="The
-Linux Documentation Project">.
-
-<sect>Introduction
-<p>
-With available IP network numbers rapidly becoming an endangered
-species, efficient use of this increasingly scarce resource is
-important.
-
-<p>
-This document describes how to split a single IP network number up so
-that it can be used on several different networks.
-
-<p>
-This document concentrates on C Class IP network numbers - but the
-principles apply to A and B class networks as well.
-
-<sect1>Other sources of information
-<p>
-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:-
-
-<itemize>
-
-<item><url url="http://sunsite.unc.edu/LDP/LDP/nag/nag.html" name="The
-Linux Network Administrators Guide">.
-
-<item><url url="http://linuxwww.db.erau.edu/SAG/" name="The Linux System
-Administration Guide">.
-
-<item><url url="http://www.ora.com/catalog/tcp/noframes.html"
-name="TCP/IP Network Administration by Craig Hunt, published by O'Reilly
-and Associates">.
-
-</itemize>
-
-<sect>The Anatomy of IP numbers
-<p>
-Before diving into the delight of sub-networking, we need to establish
-some IP number basics.
-
-<sect1>IP numbers belong to Interfaces - <bf/NOT/ hosts!
-<p>
-First of all, let's clear up a basic cause of misunderstanding - IP
-numbers are <bf/not/ assigned to hosts. IP numbers are assigned to
-network interfaces on hosts.
-
-<p>
-Eh - what's that?
-
-<p>
-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.
-
-<p>
-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 network!
-
-<p>
-Despite this, most people refer to <em/host addresses/ when referring to an
-IP number. Just remember, this is simply shorthand for <em/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.
-
-<sect1>IP Numbers as &dquot;Dotted Quads&dquot;
-<p>
-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
-
-<tscreen><verb>
- 192.168.1.24
-</verb></tscreen>
-
-is an example of this - 4 (decimal) numbers separated by (.) dots.
-
-<p>
-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).
-
-<p>
-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'.
-
-<sect1>Classes of IP Networks
-<p>
-There are three classes of IP numbers
-
-<itemize>
-
-<item>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
-network.<newline>
-Class A addresses <bf/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
-interfaces.
-
-<newline><newline>
-
-However, the networks 0.0.0.0 (known as the default route) and 127.0.0.0
-(the loop back network) have special meanings and are not available for
-use to identify networks. So there are only 126 <em/available/ A class
-network numbers.
-
-<item>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.
-
-<item>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).
-</itemize>
-
-In summary:
-
-<tscreen><verb>
- Network class Usable range of first byte values (decimal)
- A 1 to 126
- B 128 to 191
- C 192 to 254
-</verb></tscreen>
-
-<p>
-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:-
-
-<itemize>
-<item>One A Class Network<newline>
-10.0.0.0
-<item>16 B Class Networks<newline>
-172.16.0.0 - 172.31.0.0
-<item>256 C Class Networks
-192.168.0.0 - 192.168.255.0
-</itemize>
-
-<p>
-You will note that this document uses these sequences throughout to avoid
-confusion with 'real' networks and hosts.
-
-<sect1>Network numbers, interface addresses and broadcast addresses
-<p>
-IP numbers can have three possible meanings:-
-
-<itemize>
-
-<item>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);
-
-<item>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).
-
-<item>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 <bf/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.
-
-</itemize>
-
-<p>
-In summary and to clarify things
-
-<tscreen><verb>
-For an A class network...
-(one byte of network address space followed by three bytes of host
-address space)
-
- 10.0.0.0 is an A Class network number because all the host
- bits of the address space are 0
- 10.0.1.0 is a host address on this network
- 10.255.255.255 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)
-
- 172.17.0.0 is a B Class network number
- 172.17.0.1 is a host address on this network
- 172.17.255.255 is the network broadcast address
-
-For a C Class network...
-(three bytes of network address space followed by one byte of host
-address space)
-
- 192.168.3.0 is a C Class network number
- 192.168.3.42 is a host address on this network
- 192.168.3.255 is the network broadcast address
-</verb></tscreen>
-
-<p>
-Almost all IP network numbers remaining available for allocation at
-present are C Class addresses.
-
-<sect1>The network mask
-<p>
-The network mask is more properly called the subnetwork mask. However,
-it is generally referred to as the network mask.
-
-<p>
-It is the network mask and its implications on how IP addresses are
-interpreted <em/locally/ on an IP network segment that concerns us most
-here, as this determines what (if any) sub-networking occurs.
-
-<p>
-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:-
-
-<itemize>
-<item>A Class network mask: 255.0.0.0
-<item>B Class network mask: 255.255.0.0
-<item>C Class network mask: 255.255.255.0
-</itemize>
-
-<p>
-There are two important things to remember about the network mask:-
-<itemize>
-<item>The network mask affects only the <bf/local/ interpretation of
-<bf/local/ IP numbers (where local means on this particular network segment);
-<item>The network mask is <bf/not/ an IP number - it is used to modify
-how local IP numbers are interpreted locally.
-</itemize>
-
-<sect>What are subnets?
-<p>
-A subnet is a way of taking a single IP network address and <bf/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.
-
-<p>
-The important word here is <bf/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 <bf/local/ configuration
-and is invisible to the rest of the world.
-
-<sect>Why subnetwork?
-<p>
-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.
-
-<p>
-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).
-
-<p>
-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.
-
-<p>
-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!
-
-<p>
-Other reasons for sub-networking are:-
-<itemize>
-<item>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.
-<newline><newline>
-This is in fact now very commonly done by ISPs who wish to give their
-permanently connected clients with local networks static IP numbers.
-
-<item>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;
-<item>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)!
-<item>You have equipment which uses incompatible networking technologies
-and need to interconnect them (as mentioned above).
-</itemize>
-
-<sect>How to subnetwork a IP network number
-<p>
-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:-
-
-<itemize>
-<item>Set up the physical connectivity (network wiring and network
-interconnections - such as routers;
-<item>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.
-<item>Calculate the appropriate network mask and network addresses;
-<item>Give each interface on each network its own IP address and the
-appropriate network mask;
-<item>Set up the routes on the routers and the appropriate gateways,
-routes and/or default routes on the networked devices;
-<item>Test the system, fix problems and then relax!
-</itemize>
-
-<p>
-For the purpose of this example, we will assume we are sub-networking a single C
-class network number: 192.168.1.0
-
-<p>
-This provides for a maximum of 254 connected interfaces (hosts), plus
-the obligatory network number (192.168.1.0) and broadcast address
-(192.168.1.255).
-
-<sect1>Setting up the physical connectivity
-<p>
-You will need to install the correct cabling infrastructure for all the
-devices you wish to interconnect designed to meet your physical layout.
-
-<p>
-You will also need a mechanism to interconnect the various segments
-together (routers, media converters etc.).
-
-<p>
-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).
-
-<sect1>Subnetwork sizing
-<p>
-There is a play off between the number of subnetworks you create and 'wasted'
-IP numbers.
-
-<p>
-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.
-
-<p>
-So, by sub-networking an IP network into two separate subnetworks, there
-are now <bf/two/ network addresses and <bf/two/ broadcast addresses -
-increasing the 'unusable' interface (host) addresses; creating 4
-subnetworks creates <bf/eight/ unusable interface (host) addresses and
-so on.
-
-<p>
-In fact the smallest usable subnetwork consists of 4 IP numbers:-
-<itemize>
-<item>Two usable IP interface numbers - one for the router interface on
-that network and one for the single host on that network.
-<item>One network number.
-<item>One broadcast address.
-</itemize>
-
-<p>
-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-networking.
-
-<p>
-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).
-
-<p>
-So be realistic about designing your network design - you want the
-<bf/minimum/ number of separate local networks that is consistent with
-management, physical, equipment and security constraints!
-
-<sect1>Calculating the subnetwork mask and network numbers
-<p>
-The network mask is what performs all the <bf/local/ magic of dividing
-an IP network into subnetworks.
-
-<p>
-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'.
-
-<p>
-So, for the three classes of IP networks, the standard network masks
-are:-
-<itemize>
-<item>Class A (8 network bits) : 255.0.0.0
-<item>Class B (16 network bits): 255.255.0.0
-<item>Class C (24 network bits): 255.255.255.0
-</itemize>
-
-<p>
-The way sub-networking operates is to <em/borrow/ one or more of the
-available host bits and make then make interfaces <bf/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'.
-
-<p>
-For a C Class address, this would result in a netmask of
-<newline>
-11111111.11111111.11111111.10000000
-<newline>
-or 255.255.255.128
-
-<p>
-For our C Class network number of 192.168.1.0, these are some of the
-sub-networking options you have:-
-
-<code>
-No of No of
-subnets Hosts/net netmask
-2 126 255.255.255.128 (11111111.11111111.11111111.10000000)
-4 62 255.255.255.192 (11111111.11111111.11111111.11000000)
-8 30 255.255.255.224 (11111111.11111111.11111111.11100000)
-16 14 255.255.255.240 (11111111.11111111.11111111.11110000)
-32 6 255.255.255.248 (11111111.11111111.11111111.11111000)
-64 2 255.255.255.252 (11111111.11111111.11111111.11111100)
-</code>
-
-<p>
-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 <em/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).
-
-<p>
-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 <em/final/ (host part) we have:-
-
-<code>
-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
-</code>
-
-<p>
-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.
-
-<p> With this information, you are now in a position to assign host and
-network IP numbers and netmasks.
-
-<sect>Routing
-<p>
-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
-
-<code>
- cat /proc/ksyms | grep ip_forward
-</code>
-
-<p>
-You should get back something like...
-<code>
-00141364 ip_forward_Rf71ac834
-</code>
-
-<p>
-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.
-
-<p>
-For the sake of this example, let us assume that you have decided to
-subnetwork you C class IP network number 192.168.1.0 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.
-
-<p>
-These are :-
-<code>
-Network Broadcast Netmask Hosts
-192.168.1.0 192.168.1.63 255.255.255.192 62
-192.168.1.64 192.168.1.127 255.255.255.192 62
-192.168.1.128 192.168.1.255 255.255.255.128 124 (see note)
-</code>
-
-<p>
-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 192.168.1.192 as the <em/network/ address of the 'non-existent'
-subnetwork. Similarly, they will interpret 192.168.1.191
-as the broadcast address of the 'non-existent' subnetwork.
-
-<p>
-So, if you use 192.168.1.191 or 192 as host addresses on the third
-network, then machines on the two smaller networks will not be able to
-communicate with them.
-
-<p>
-This illustrates an important point with subnetworks - the usable
-addresses are determined by the <bf/SMALLEST/ subnetwork in that address
-space.
-
-<sect1>The routing tables
-<p>
-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.
-
-<p>
-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
-
-<code>
-Interface IP Address Netmask
-eth0 192.168.1.1 255.255.255.192
-eth1 192.168.1.65 255.255.255.192
-eth2 192.168.1.129 255.255.255.128
-</code>
-
-<p>
-The routing it would establish would be
-
-<code>
-Destination Gateway Genmask Iface
-192.168.1.0 0.0.0.0 255.255.255.192 eth0
-192.168.1.64 0.0.0.0 255.255.255.192 eth1
-192.168.1.128 0.0.0.0 255.255.255.128 eth2
-</code>
-
-<p>
-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 network.
-
-
-<p>
-Robert Hart
-Melbourne, Australia March 1997.
-
-</article>
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-<head>
-
-<title>KickStart HOWTO
-
-
-
-
-
-KickStart HOWTO
-
-This document has been removed at the request of the author, Martin
-Hamilton, because it has been replaced by RedHat's own documentation.
-
-For more information on KickStart Installations and Configurations, you
-may want to try the following Web sites:
-
-
-
-
-