ACTUALLY GUYS THIS POST IS NOT BY ME.I FOUND IT ON CISCO'S SITE SO I DECIDED TO PUT THIS POST ON MY BLOG :)
This document gives you basic information needed in order to configure
your router for routing IP, such as how addresses are broken down and how
subnetting works. You learn how to assign each interface on the router an IP
address with a unique subnet. There are many examples to help tie everything
together.
Cisco recommends that you have knowledge of these topics:
-
Basic understanding of binary and decimal
numbers.
If definitions are helpful to you, use these vocabulary terms to get
you started:
-
Address—The unique number ID assigned to one host or
interface in a network.
-
Subnet—A portion of a network sharing a particular
subnet address.
-
Subnet mask—A 32-bit combination used to describe
which portion of an address refers to the subnet and which part refers to the
host.
-
Interface—A network connection.
If you have already received your legitimate address(es) from the
Internet Network Information Center (InterNIC), you are ready to begin. If you
do not plan to connect to the Internet, Cisco strongly suggests that you use
reserved addresses from
RFC 1918 .
An IP address is an address used in order to uniquely identify a device
on an IP network. The address is made up of 32 binary bits, which can be
divisible into a network portion and host portion with the help of a subnet
mask. The 32 binary bits are broken into four octets (1 octet = 8 bits). Each
octet is converted to decimal and separated by a period (dot). For this reason,
an IP address is said to be expressed in dotted decimal format (for example,
172.16.81.100). The value in each octet ranges from 0 to 255 decimal, or
00000000 - 11111111 binary.
Here is how binary octets convert to decimal: The right most bit, or
least significant bit, of an octet holds a value of
2
0. The bit just to the left of that holds a value
of 2
1. This continues until the left-most bit, or
most significant bit, which holds a value of 2
7. So
if all binary bits are a one, the decimal equivalent would be 255 as shown
here:
1 1 1 1 1 1 1 1
128 64 32 16 8 4 2 1 (128+64+32+16+8+4+2+1=255)
Here is a sample octet conversion when not all of the bits are set to
1.
0 1 0 0 0 0 0 1
0 64 0 0 0 0 0 1 (0+64+0+0+0+0+0+1=65)
And this is sample shows an IP address represented in both binary and
decimal.
10. 1. 23. 19 (decimal)
00001010.00000001.00010111.00010011 (binary)
These octets are broken down to provide an addressing scheme that can
accommodate large and small networks. There are five different classes of
networks, A to E. This document focuses on addressing classes A to C, since
classes D and E are reserved and discussion of them is beyond the scope of this
document.
Note: Also note that the terms "Class A, Class B" and so on are used in
this document to help facilitate the understanding of IP addressing and
subnetting. These terms are rarely used in the industry anymore because of the
introduction of
classless interdomain routing
(CIDR).
Given an IP address, its class can be determined from the three
high-order bits.
Figure 1 shows the significance
in the three high order bits and the range of addresses that fall into each
class. For informational purposes, Class D and Class E addresses are also
shown.
FIGURE 1
In a Class A address, the first octet is the network portion, so the
Class A example in Figure 1 has a major network
address of 1.0.0.0 - 127.255.255.255. Octets 2, 3, and 4 (the next 24 bits) are
for the network manager to divide into subnets and hosts as he/she sees fit.
Class A addresses are used for networks that have more than 65,536 hosts
(actually, up to 16777214 hosts!).
In a Class B address, the first two octets are the network portion, so
the Class B example in
Figure 1 has a major
network address of 128.0.0.0 - 191.255.255.255. Octets 3 and 4 (16 bits) are
for local subnets and hosts. Class B addresses are used for networks that have
between 256 and 65534 hosts.
In a Class C address, the first three octets are the network portion.
The Class C example in
Figure 1 has a major
network address of 192.0.0.0 - 233.255.255.255. Octet 4 (8 bits) is for local
subnets and hosts - perfect for networks with less than 254 hosts.
A network mask helps you know which portion of the address identifies
the network and which portion of the address identifies the node. Class A, B,
and C networks have default masks, also known as natural masks, as shown
here:
Class A: 255.0.0.0
Class B: 255.255.0.0
Class C: 255.255.255.0
An IP address on a Class A network that has not been subnetted would
have an address/mask pair similar to: 8.20.15.1 255.0.0.0. To see how the mask
helps you identify the network and node parts of the address, convert the
address and mask to binary numbers.
8.20.15.1 = 00001000.00010100.00001111.00000001
255.0.0.0 = 11111111.00000000.00000000.00000000
Once you have the address and the mask represented in binary, then
identifying the network and host ID is easier. Any address bits which have
corresponding mask bits set to 1 represent the network ID. Any address bits
that have corresponding mask bits set to 0 represent the node ID.
8.20.15.1 = 00001000.00010100.00001111.00000001
255.0.0.0 = 11111111.00000000.00000000.00000000
-----------------------------------
net id | host id
netid = 00001000 = 8
hostid = 00010100.00001111.00000001 = 20.15.1
Subnetting allows you to create multiple logical networks that exist
within a single Class A, B, or C network. If you do not subnet, you are only
able to use one network from your Class A, B, or C network, which is
unrealistic.
Each data link on a network must have a unique network ID, with every
node on that link being a member of the same network. If you break a major
network (Class A, B, or C) into smaller subnetworks, it allows you to create a
network of interconnecting subnetworks. Each data link on this network would
then have a unique network/subnetwork ID. Any device, or gateway, connecting
n networks/subnetworks has
n distinct
IP addresses, one for each network / subnetwork that it interconnects.
In order to subnet a network, extend the natural mask using some of the
bits from the host ID portion of the address to create a subnetwork ID. For
example, given a Class C network of 204.17.5.0 which has a natural mask of
255.255.255.0, you can create subnets in this manner:
204.17.5.0 - 11001100.00010001.00000101.00000000
255.255.255.224 - 11111111.11111111.11111111.11100000
--------------------------|sub|----
By extending the mask to be 255.255.255.224, you have taken three bits
(indicated by "sub") from the original host portion of the address and used
them to make subnets. With these three bits, it is possible to create eight
subnets. With the remaining five host ID bits, each subnet can have up to 32
host addresses, 30 of which can actually be assigned to a device
since host ids of all zeros or all ones are not allowed
(it is very important to remember this). So, with this in mind, these subnets
have been created.
204.17.5.0 255.255.255.224 host address range 1 to 30
204.17.5.32 255.255.255.224 host address range 33 to 62
204.17.5.64 255.255.255.224 host address range 65 to 94
204.17.5.96 255.255.255.224 host address range 97 to 126
204.17.5.128 255.255.255.224 host address range 129 to 158
204.17.5.160 255.255.255.224 host address range 161 to 190
204.17.5.192 255.255.255.224 host address range 193 to 222
204.17.5.224 255.255.255.224 host address range 225 to 254
Note: There are two ways to denote these masks. First, since you are using
three bits more than the "natural" Class C mask, you can denote these addresses
as having a 3-bit subnet mask. Or, secondly, the mask of 255.255.255.224 can
also be denoted as /27 as there are 27 bits that are set in the mask. This
second method is used with
CIDR. With this method,
one of these networks can be described with the notation prefix/length. For
example, 204.17.5.32/27 denotes the network 204.17.5.32 255.255.255.224. When
appropriate the prefix/length notation is used to denote the mask throughout
the rest of this document.
The network subnetting scheme in this section allows for eight subnets,
and the network might appear as:
FIGURE 2
Notice that each of the routers in Figure
2 is attached to four subnetworks, one subnetwork is common to both
routers. Also, each router has an IP address for each subnetwork to which it is
attached. Each subnetwork could potentially support up to 30 host
addresses.
This brings up an interesting point. The more host bits you use for a
subnet mask, the more subnets you have available. However, the more subnets
available, the less host addresses available per subnet. For example, a Class C
network of 204.17.5.0 and a mask of 255.255.255.224 (/27) allows you to have
eight subnets, each with 32 host addresses (30 of which could be assigned to
devices). If you use a mask of 255.255.255.240 (/28), the break down is:
204.17.5.0 - 11001100.00010001.00000101.00000000
255.255.255.240 - 11111111.11111111.11111111.11110000
--------------------------|sub |---
Since you now have four bits to make subnets with, you only have four
bits left for host addresses. So in this case you can have up to 16 subnets,
each of which can have up to 16 host addresses (14 of which can be assigned to
devices).
Take a look at how a Class B network might be subnetted. If you have
network 172.16.0.0 ,then you know that its natural mask is 255.255.0.0 or
172.16.0.0/16. Extending the mask to anything beyond 255.255.0.0 means you are
subnetting. You can quickly see that you have the ability to create a lot more
subnets than with the Class C network. If you use a mask of 255.255.248.0
(/21), how many subnets and hosts per subnet does this allow for?
172.16.0.0 - 10101100.00010000.00000000.00000000
255.255.248.0 - 11111111.11111111.11111000.00000000
-----------------| sub |-----------
You are using five bits from the original host bits for subnets. This
allows you to have 32 subnets (2
5). After using the
five bits for subnetting, you are left with 11 bits for host addresses. This
allows each subnet so have 2048 host addresses
(2
11), 2046 of which could be assigned to
devices.
Note: In the past, there were limitations to the use of a subnet 0 (all
subnet bits are set to zero) and all ones subnet (all subnet bits set to one).
Some devices would not allow the use of these subnets. Cisco Systems devices
allow the use of these subnets when the
ip subnet
zero command is configured.
Now that you have an understanding of subnetting, put this knowledge to
use. In this example, you are given two address / mask combinations, written
with the prefix/length notation, which have been assigned to two devices. Your
task is to determine if these devices are on the same subnet or different
subnets. You can do this by using the address and mask of each device to
determine to which subnet each address belongs.
DeviceA: 172.16.17.30/20
DeviceB: 172.16.28.15/20
Determining the Subnet for DeviceA:
172.16.17.30 - 10101100.00010000.00010001.00011110
255.255.240.0 - 11111111.11111111.11110000.00000000
-----------------| sub|------------
subnet = 10101100.00010000.00010000.00000000 = 172.16.16.0
Looking at the address bits that have a corresponding mask bit set to
one, and setting all the other address bits to zero (this is equivalent to
performing a logical "AND" between the mask and address), shows you to which
subnet this address belongs. In this case, DeviceA belongs to subnet
172.16.16.0.
Determining the Subnet for DeviceB:
172.16.28.15 - 10101100.00010000.00011100.00001111
255.255.240.0 - 11111111.11111111.11110000.00000000
-----------------| sub|------------
subnet = 10101100.00010000.00010000.00000000 = 172.16.16.0
From these determinations, DeviceA and DeviceB have addresses that are
part of the same subnet.
Given the Class C network of 204.15.5.0/24, subnet the network in order
to create the network in
Figure 3 with the
host requirements shown.
FIGURE 3
Looking at the network shown in Figure
3, you can see that you are required to create five subnets. The largest
subnet must support 28 host addresses. Is this possible with a Class C network?
and if so, then how?
You can start by looking at the subnet requirement. In order to create
the five needed subnets you would need to use three bits from the Class C host
bits. Two bits would only allow you four subnets
(2
2).
Since you need three subnet bits, that leaves you with five bits for
the host portion of the address. How many hosts does this support?
2
5 = 32 (30 usable). This meets the
requirement.
Therefore you have determined that it is possible to create this
network with a Class C network. An example of how you might assign the
subnetworks is:
netA: 204.15.5.0/27 host address range 1 to 30
netB: 204.15.5.32/27 host address range 33 to 62
netC: 204.15.5.64/27 host address range 65 to 94
netD: 204.15.5.96/27 host address range 97 to 126
netE: 204.15.5.128/27 host address range 129 to 158
In all of the previous examples of subnetting, notice that the same
subnet mask was applied for all the subnets. This means that each subnet has
the same number of available host addresses. You can need this in some cases,
but, in most cases, having the same subnet mask for all subnets ends up wasting
address space. For example, in the
Sample Exercise
2 section, a class C network was split into eight equal-size subnets;
however, each subnet did not utilize all available host addresses, which
results in wasted address space.
Figure 4
illustrates this wasted address space.
FIGURE 4
Figure 4 illustrates that of the
subnets that are being used, NetA, NetC, and NetD have a lot of unused host
address space. It is possible that this was a deliberate design accounting for
future growth, but in many cases this is just wasted address space due to the
fact that the same subnet mask is being used for all the subnets.
Variable Length Subnet Masks (VLSM) allows you to use different masks
for each subnet, thereby using address space efficiently.
Given the same network and requirements as in
Sample Exercise 2 develop a subnetting scheme with the
use of VLSM, given:
netA: must support 14 hosts
netB: must support 28 hosts
netC: must support 2 hosts
netD: must support 7 hosts
netE: must support 28 host
Determine what mask allows the required number of hosts.
netA: requires a /28 (255.255.255.240) mask to support 14 hosts
netB: requires a /27 (255.255.255.224) mask to support 28 hosts
netC: requires a /30 (255.255.255.252) mask to support 2 hosts
netD*: requires a /28 (255.255.255.240) mask to support 7 hosts
netE: requires a /27 (255.255.255.224) mask to support 28 hosts
* a /29 (255.255.255.248) would only allow 6 usable host addresses
therefore netD requires a /28 mask.
The easiest way to assign the subnets is to assign the largest first.
For example, you can assign in this manner:
netB: 204.15.5.0/27 host address range 1 to 30
netE: 204.15.5.32/27 host address range 33 to 62
netA: 204.15.5.64/28 host address range 65 to 78
netD: 204.15.5.80/28 host address range 81 to 94
netC: 204.15.5.96/30 host address range 97 to 98
This can be graphically represented as shown in Figure
5:
FIGURE 5
Figure 5 illustrates how using VLSM
helped save more than half of the address space.
Classless Interdomain Routing (CIDR) was introduced to improve both
address space utilization and routing scalability in the Internet. It was
needed because of the rapid growth of the Internet and growth of the IP routing
tables held in the Internet routers.
CIDR moves way from the traditional IP classes (Class A, Class B, Class
C, and so on). In CIDR , an IP network is represented by a prefix, which is an
IP address and some indication of the length of the mask. Length means the
number of left-most contiguous mask bits that are set to one. So network
172.16.0.0 255.255.0.0 can be represented as 172.16.0.0/16. CIDR also depicts a
more hierarchical Internet architecture, where each domain takes its IP
addresses from a higher level. This allows for the summarization of the domains
to be done at the higher level. For example, if an ISP owns network
172.16.0.0/16, then the ISP can offer 172.16.1.0/24, 172.16.2.0/24, and so on
to customers. Yet, when advertising to other providers, the ISP only needs to
advertise 172.16.0.0/16.
