193 lines
8.2 KiB
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193 lines
8.2 KiB
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<TITLE>IP Sub-Networking Mini-Howto: How to subnetwork a IP network number</TITLE>
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<LINK HREF="IP-Subnetworking-7.html" REL=next>
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<LINK HREF="IP-Subnetworking.html#toc6" REL=contents>
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<A HREF="IP-Subnetworking-7.html">Next</A>
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<A HREF="IP-Subnetworking.html#toc6">Contents</A>
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<HR>
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<H2><A NAME="s6">6. How to subnetwork a IP network number</A></H2>
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<P>Having decided that you need to subnetwork your IP network number, how
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do you go about it? The following is an overview of the steps which will
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then be explained in detail:-
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<P>
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<UL>
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<LI>Set up the physical connectivity (network wiring and network
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interconnections - such as routers;</LI>
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<LI>Decide how big/small each subnetwork needs to be in terms of the
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number of devices that will connect to it - ie how many usable IP
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numbers are required for each individual segment.</LI>
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<LI>Calculate the appropriate network mask and network addresses;</LI>
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<LI>Give each interface on each network its own IP address and the
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appropriate network mask;</LI>
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<LI>Set up the routes on the routers and the appropriate gateways,
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routes and/or default routes on the networked devices;</LI>
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<LI>Test the system, fix problems and then relax!</LI>
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</UL>
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<P>
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<P>For the purpose of this example, we will assume we are sub-networking a single C
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class network number: 192.168.1.0
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<P>
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<P>This provides for a maximum of 254 connected interfaces (hosts), plus
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the obligatory network number (192.168.1.0) and broadcast address
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(192.168.1.255).
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<P>
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<H2><A NAME="ss6.1">6.1 Setting up the physical connectivity</A>
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</H2>
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<P>You will need to install the correct cabling infrastructure for all the
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devices you wish to interconnect designed to meet your physical layout.
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<P>
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<P>You will also need a mechanism to interconnect the various segments
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together (routers, media converters etc.).
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<P>
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<P>A detailed discussion of this is obviously impossible here. Should you
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need help, there are network design/installation consultants around who
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provide this sort of service. Free advice is also available on a number of
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Usenet news groups (such as comp.os.linux.networking).
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<P>
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<H2><A NAME="ss6.2">6.2 Subnetwork sizing</A>
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</H2>
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<P>There is a play off between the number of subnetworks you create and 'wasted'
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IP numbers.
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<P>
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<P>Every individual IP network has two addresses unusable as interface
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(host) addresses - the network IP number itself and the broadcast
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address. When you subnetwork, each subnetwork requires its own, unique
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IP network number and broadcast address - and these have to be valid
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addresses from within the range provided by the IP network that you are
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sub-networking.
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<P>
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<P>So, by sub-networking an IP network into two separate subnetworks, there
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are now <B>two</B> network addresses and <B>two</B> broadcast addresses -
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increasing the 'unusable' interface (host) addresses; creating 4
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subnetworks creates <B>eight</B> unusable interface (host) addresses and
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so on.
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<P>
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<P>In fact the smallest usable subnetwork consists of 4 IP numbers:-
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<UL>
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<LI>Two usable IP interface numbers - one for the router interface on
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that network and one for the single host on that network.</LI>
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<LI>One network number.</LI>
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<LI>One broadcast address.</LI>
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</UL>
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<P>
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<P>Quite why one would want to create such a small network is another
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question! With only a single host on the network, any network
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communication must go out to another network. However, the example does
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serve to show the law of diminishing returns that applies to
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sub-networking.
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<P>
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<P>In principle, you can only divide your IP network number into 2^n (where
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n is one less that the number of host bits in your IP network number)
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equally sized subnetworks (you can subnetwork a subnetwork and combine
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subnetworks however).
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<P>
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<P>So be realistic about designing your network design - you want the
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<B>minimum</B> number of separate local networks that is consistent with
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management, physical, equipment and security constraints!
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<P>
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<H2><A NAME="ss6.3">6.3 Calculating the subnetwork mask and network numbers</A>
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</H2>
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<P>The network mask is what performs all the <B>local</B> magic of dividing
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an IP network into subnetworks.
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<P>
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<P>The network mask for an un-sub-networked IP network number is simply a
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dotted quad which has all the 'network bits' of the network number
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set to '1' and all the host bits set to '0'.
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<P>
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<P>So, for the three classes of IP networks, the standard network masks
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are:-
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<UL>
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<LI>Class A (8 network bits) : 255.0.0.0</LI>
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<LI>Class B (16 network bits): 255.255.0.0</LI>
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<LI>Class C (24 network bits): 255.255.255.0</LI>
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</UL>
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<P>
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<P>The way sub-networking operates is to <EM>borrow</EM> one or more of the
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available host bits and make then make interfaces <B>locally</B> interpret
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these borrowed bits as part of the network bits. So to divide a network
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number into two subnetworks, we would borrow one host bit by setting the
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appropriate bit in the network mask of the first (normal) host bit to '1'.
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<P>
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<P>For a C Class address, this would result in a netmask of<BR>
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11111111.11111111.11111111.10000000<BR>
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or 255.255.255.128
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<P>
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<P>For our C Class network number of 192.168.1.0, these are some of the
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sub-networking options you have:-
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<P>
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<HR>
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<PRE>
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No of No of
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subnets Hosts/net netmask
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2 126 255.255.255.128 (11111111.11111111.11111111.10000000)
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4 62 255.255.255.192 (11111111.11111111.11111111.11000000)
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8 30 255.255.255.224 (11111111.11111111.11111111.11100000)
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16 14 255.255.255.240 (11111111.11111111.11111111.11110000)
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32 6 255.255.255.248 (11111111.11111111.11111111.11111000)
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64 2 255.255.255.252 (11111111.11111111.11111111.11111100)
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</PRE>
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<HR>
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<P>
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<P>In principle, there is absolutely no reason to follow the above way of
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subnetworking where network mask bits are added from the most
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significant host bit to the least significant host bit. However, if you
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do not do it this way, the resulting IP numbers will be in a <EM>very</EM>
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odd sequence! This makes it extremely difficult for us humans to decide
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to which subnetwork an IP number belongs as we are not too good at thinking
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in binary (computers on the other hand are and will use whatever scheme
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you tell them with equal equanimity).
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<P>
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<P>Having decided on the appropriate netmask, you then need to work out
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what the various Network and broadcast addresses are - and the IP number
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range for each of these networks. Again, considering only a C Class IP
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Network number and listing only the <EM>final</EM> (host part) we have:-
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<P>
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<HR>
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<PRE>
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Netmask Subnets Network B'cast MinIP MaxIP Hosts Total Hosts
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--------------------------------------------------------------------------
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128 2 0 127 1 126 126
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128 255 129 254 126 252
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192 4 0 63 1 62 62
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64 127 65 126 62
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128 191 129 190 62
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192 255 193 254 62 248
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224 8 0 31 1 30 30
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32 63 33 62 30
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64 95 65 94 30
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96 127 97 126 30
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128 159 129 158 30
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160 191 161 190 30
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192 223 193 222 30
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224 255 225 254 30 240
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</PRE>
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<HR>
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<P>
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<P>As can be seen, there is a very definite sequence to these numbers,
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which make them fairly easy to check. The 'downside' of sub-networking is
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also visible in terms of the reducing total number of available host
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addresses as the number of subnetworks increases.
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<P>
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<P> With this information, you are now in a position to assign host and
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network IP numbers and netmasks.
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<P>
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<HR>
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<A HREF="IP-Subnetworking-7.html">Next</A>
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<A HREF="IP-Subnetworking-5.html">Previous</A>
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<A HREF="IP-Subnetworking.html#toc6">Contents</A>
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