220 lines
4.0 KiB
HTML
220 lines
4.0 KiB
HTML
<HTML
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><HEAD
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><TITLE
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>Creating Subnets</TITLE
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><META
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NAME="GENERATOR"
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CONTENT="Modular DocBook HTML Stylesheet Version 1.57"><LINK
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REL="HOME"
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TITLE="Linux Network Administrators Guide"
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HREF="index.html"><LINK
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REL="UP"
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TITLE="Configuring TCP/IP Networking"
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HREF="x-087-2-iface.html"><LINK
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REL="PREVIOUS"
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TITLE="Assigning IP Addresses"
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HREF="x-087-2-iface.addresses.html"><LINK
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REL="NEXT"
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TITLE="Writing hosts and networks Files"
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HREF="x-087-2-iface.simple-resolv.html"></HEAD
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><TR
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COLSPAN="3"
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>Linux Network Administrators Guide</TH
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></TR
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><TR
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><TD
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WIDTH="10%"
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>Prev</A
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></TD
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><TD
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WIDTH="80%"
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ALIGN="center"
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VALIGN="bottom"
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>Chapter 5. Configuring TCP/IP Networking</TD
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><TD
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WIDTH="10%"
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VALIGN="bottom"
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><DIV
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CLASS="SECT1"
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><H1
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CLASS="SECT1"
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><A
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NAME="X-087-2-CREATE.SUBNETS"
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>5.5. Creating Subnets</A
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></H1
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><P
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>To operate several Ethernets (or other networks, once a driver is
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available), you have to split your network into subnets. Note that
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subnetting is required only if you have more than one
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<I
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CLASS="EMPHASIS"
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>broadcast network</I
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>—point-to-point links
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don't count. For instance, if you have one Ethernet, and one or more
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SLIP links to the outside world, you don't need to subnet your
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network. This is explained in more detail in <A
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HREF="x-087-2-slip.html"
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>Chapter 7</A
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>.</P
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><P
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>To accommodate the two Ethernets, the Brewery's network manager decides
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to use 8 bits of the host part as additional subnet bits. This
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leaves another 8 bits for the host part, allowing for 254 hosts on
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each of the subnets. She then assigns subnet number 1 to the brewery,
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and gives the winery number 2. Their respective network addresses are
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thus <SPAN
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CLASS="SYSTEMITEM"
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>172.16.1.0</SPAN
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> and
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<SPAN
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CLASS="SYSTEMITEM"
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>172.16.2.0</SPAN
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>. The subnet mask is
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<SPAN
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CLASS="SYSTEMITEM"
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>255.255.255.0</SPAN
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>.</P
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><P
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><SPAN
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CLASS="SYSTEMITEM"
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>vlager</SPAN
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>, which is the gateway between
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the two networks, is assigned a host number of 1 on both of them, which gives
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it the IP addresses <SPAN
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CLASS="SYSTEMITEM"
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>172.16.1.1</SPAN
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> and
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<SPAN
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CLASS="SYSTEMITEM"
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>172.16.2.1</SPAN
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>, respectively.</P
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><P
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>Note that in this example we are using a class B network to keep things
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simple, but a class C network would be more realistic. With the new networking
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code, subnetting is not limited to byte boundaries, so even a class C
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network may be split into several subnets. For instance, you could use two
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bits of the host part for the netmask, giving you 4 possible subnets
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with 64 hosts on each.<A
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NAME="X-087-2-FNTC02"
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HREF="#FTN.X-087-2-FNTC02"
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>[1]</A
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></P
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></DIV
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><H3
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CLASS="FOOTNOTES"
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>Notes</H3
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><TABLE
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CLASS="FOOTNOTES"
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WIDTH="100%"
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><TR
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><TD
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ALIGN="LEFT"
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VALIGN="TOP"
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WIDTH="5%"
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><A
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NAME="FTN.X-087-2-FNTC02"
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HREF="x-087-2-create.subnets.html#X-087-2-FNTC02"
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>[1]</A
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></TD
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><TD
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ALIGN="LEFT"
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VALIGN="TOP"
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WIDTH="95%"
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><P
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>The first number on each subnet is the subnetwork address, and the last number
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on each subnet is reserved as the broadcast address, so it's actually 62 hosts
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per subnet.</P
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></TD
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></TR
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><A
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HREF="x-087-2-iface.addresses.html"
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>Prev</A
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><TD
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WIDTH="33%"
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>Assigning IP Addresses</TD
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><TD
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WIDTH="34%"
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>Writing hosts and networks Files</TD
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