Contents
Introduction
Prerequisites
Requirements
Components Used
Additional
Information
Conventions
Understanding IP Addresses
Network Masks
Understanding Subnetting
Examples
Sample Exercise
1
Sample Exercise
2
VLSM Example
VLSM Example
CIDR
Appendix
Sample Config
Host/Subnet
Quantities Table
Related Information
Introduction
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.
Prerequisites
Requirements
Cisco recommends that you have knowledge of these topics:
•
Basic understanding of binary and decimal numbers.
Components Used
This document is not restricted to specific software and
hardware versions.
Additional Information
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 .
Conventions
Refer to Cisco Technical Tips Conventions for more
information on document conventions.
Understanding IP Addresses
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 20. The
bit just to the left of that holds a value of 21. This continues
until the left−most bit, or most significant bit, which holds a value of 27.
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 − 223.255.255.255. Octet
4 (8 bits) is for local subnets and hosts − perfect for networks with less than
254 hosts.
Network Masks
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
Understanding Subnetting
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 (25). 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 (211), 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 theip subnet zero
command is configured.
Examples
Sample Exercise 1
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.
Sample Exercise 2
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 (22).
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? 25
= 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
VLSM Example
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.
VLSM Example
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.
CIDR
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.
For more information on CIDR, see RFC 1518 and RFC 1519 .
Appendix
Sample Config
Routers A and B are connected via serial interface.
Router A
hostname routera !
ip routing !
int e 0 ip address 172.16.50.1
255.255.255.0 !(subnet 50) int e 1 ip address 172.16.55.1 255.255.255.0 !(subnet 55)
int t 0 ip address 172.16.60.1 255.255.255.0 !(subnet 60) int s 0 ip address 172.16.65.1 255.255.255.0 (subnet
65) !S 0 connects to router B router rip
network 172.16.0.0
Router B
hostname routerb !
ip routing !
int e 0 ip address 192.1.10.200
255.255.255.240 !(subnet 192) int e 1
ip address 192.1.10.66 255.255.255.240
!(subnet 64) int s 0
ip address 172.16.65.2 (same subnet as router A's s 0)
!Int s 0 connects to router A
router rip network
192.1.10.0 network 172.16.0.0
Host/Subnet Quantities Table
Class B Effective Effective
# bits Mask Subnets Hosts
−−−−−−−
−−−−−−−−−−−−−−− −−−−−−−−− −−−−−−−−−
1
255.255.128.0 2
32766
2
255.255.192.0 4
16382
3
255.255.224.0 8 8190
4
255.255.240.0 16 4094
5
255.255.248.0 32 2046
6
255.255.252.0 64
1022
7
255.255.254.0 128 510
8
255.255.255.0 256 254
9
255.255.255.128 512
126
10
255.255.255.192 1024 62
11
255.255.255.224 2048 30
12
255.255.255.240 4096 14
13
255.255.255.248 8192 6
14
255.255.255.252 16384 2
Class C Effective Effective
# bits Mask Subnets Hosts
−−−−−−−
−−−−−−−−−−−−−−− −−−−−−−−− −−−−−−−−−
1
255.255.255.128 2
126
2
255.255.255.192 4
62
3
255.255.255.224 8
30
4
255.255.255.240 16
14
5
255.255.255.248 32
6
6
255.255.255.252 64
2
*Subnet all zeroes and all ones
included. These might not be supported
on some legacy systems. *Host all zeroes and all ones excluded.
Related Information
• IP Subnet Calculator ( registered customers only)
• IP Routing Protocols Technology Support
• Subnet Zero and the All−Ones Subnet
• Host and Subnet Quantities
• Technical Support & Documentation −
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Updated: Mar 18, 2015 Document
ID: 13788
