Transcript CCNA v3.0 Retooling

CCNA
Variable Length Subnet Mask
(VLSM)
What is VLSM?
A Variable Length Subnet Mask (VLSM) is a means of
allocating IP addressing resources to subnets
according to their individual need rather than some
general network-wide rule.
VLSM allows an organization to use more than one
subnet mask within the same network address space.
It is often referred to as ‘subnetting a subnet’, and
can be used to maximize addressing efficiency.
Large subnets are created for addressing LANs and
small subnets are created for WAN links (a 30 bit
mask is used to create subnets with only two host).
2
Subnetting vs. VLSM
• Subnetting allows you to divide big networks
into smaller, equal-sized slices.
• VLSM allows you to divide big networks into
smaller, different-sized slices. This enables
you to make maximum use of your valuable
IP address space.
3
Addressing a Network with
Standard Subnetting
• Site A has two Ethernet networks
• Site B had one Ethernet network
• Site C had one Ethernet network
Site A
Site B
Site C
10 users
8 users
207.21.24.0 /24
25 users
25 users
How many network addresses are needed?
How many hosts are needed for the largest LAN?
How many bits need to be borrowed to address this
network?
4
Addressing a Network with
Standard Subnetting
• Site A has two Ethernet networks
• Site B had one Ethernet network
• Site C had one Ethernet network
Site A
25 users 25 users
Site B
Site C
10 users
8 users
If we borrow 3 bits from a class C address, that will give
us eight networks. Each network will have 30 usable
addresses.
It will take four network addresses to accommodate the
Ethernet networks at each site. That leaves us with four
extra networks.
There is also a point-to-point WAN connection between
each site. These two connections will take up two of the
remaining networks.
5
Addressing a Network with Standard
Subnetting
Borrowing 3 bits will meet the current needs of the
company, but it leaves little room for growth.
Each network will have 30 usable addresses, including
the point-to-point WAN links (which only require two
addresses).
Subnet # Subnet Address
0
1
2
3
4
5
6
7
207.21.24.0
207.21.24.32
207.21.24.64
207.21.24.96
207.21.24.128
207.21.24.160
207.21.24.192
207.21.24.224
Bits
Masked
/27
/27
/27
/27
/27
/27
/27
/27
207.21.24.0
Site A
25 users 25 users
Site B
10 users
Site C
8 users
6
Subnetting in a Box
To begin, in a
class C network
there are 256
addresses. When
we subnet the
address, we break
it down in to
smaller units or
subnets.
0
256 addresses
255
7
Subnetting in a Box
If we were to
borrow 1 bit, it
would break the
256 addresses in
to two parts
(networks) with
each part (subnet)
having 128
addresses.
0
128
128 addresses
128 addresses
The subnet mask
would be
255.255.255.128.
127
255
8
Subnetting in a Box
0
If we were to
borrow 2 bits, it
would break each
of these 2
networks in half
again. This would
give us 4
networks, each
with 64 addresses.
The subnet mask
would now be
255.255.255.192.
128
64 addresses
64 addresses
63
191
192
64
64 addresses
64 addresses
127
255
9
Subnetting in a Box
32
0
128
160
If we were to
borrow 3 bits, it
would break each
of these 4
32
32
32
32
networks in half
addresses addresses addresses addresses
again. This would
give us 8
networks, each
159
31
63
191
with 32 addresses.
The subnet mask
would now be
255.255.255.224.
64
192
96
32
addresses
95
32
addresses
127
224
32
addresses
223
32
addresses
255
10
Subnetting in a Box
0
If we were to
borrow 4 bits, it
16
would break each
addresses
of these 8
15
networks in half
16
again. This would
16
give us 16
addresses
networks, each
31
with 16 addresses.
The subnet mask
would now be
255.255.255.240.
64
32
128
16
addresses
16
addresses
16
addresses
95
16
addresses
239
207
208
127
191
224
16
addresses
111
16
addresses
16
addresses
159
192
112
175
176
16
addresses
63
16
addresses
16
addresses
143
144
48
79
80
16
addresses
47
96
16
addresses
160
240
16
addresses
223
16
addresses
255
11
Addressing a Network Using VLSM
• When using VLSM to subnet an address, not all of the
subnets have to be the same size.
• A different subnet mask may be applied to some of the
subnets to further subnet the address.
• In order to take advantage of VLSM, the proper routing
protocol must be selected.
• Not all routing protocols share subnetting information in
their routing table updates.
Classful Routing Protocols
(do not share subnet info)
RIP v1
IGRP
Classless Routing Protocols
(do share subnet info)
RIP v2
EIGRP
OSPF
IS-IS
12
Addressing a Network Using VLSM
To begin subnetting this network using VLSM, identify
the LAN with the largest number of hosts. Subnet the
address 207.21.24.0 /24 based on this information.
• Site A has two Ethernet networks (25 hosts each)
• Site B had one Ethernet network (10 hosts)
• Site C had one Ethernet network (8 hosts)
Subnet # Subnet Address
0
1
2
3
4
5
6
7
207.21.24.0
207.21.24.32
207.21.24.64
207.21.24.96
207.21.24.128
207.21.24.160
207.21.24.192
207.21.24.224
Bits
Masked
/27
/27
/27
/27
/27
/27
/27
/27
Site A
25 users
25 users
Site B
Site C
10 users
8 users
13
Addressing a Network Using VLSM
Subnet 1 & 2 can be used to address Site A Ethernet
networks. Subnet 5 can be subnetted to accommodate
Site B & C Ethernet networks. Subnet 6 can be subnetted
to accommodate the WAN links.
Site B
Site C
Site A
Subnet # Subnet Address
Site A
Free
Addresses
Site B & C
WAN links
0
1
2
3
4
5
6
7
207.21.24.0 /27
207.21.24.32 /27
207.21.24.64 /27
207.21.24.96 /27
207.21.24.128 /27
207.21.24.160 /27
207.21.24.192 /27
207.21.24.224 /27
25 users 25 users
10 users
8 users
Sub-subnet 0 207.21.24.160
Sub-subnet 1 207.21.24.176
/28
/28
Site B
Site C
Sub-subnet 0
Sub-subnet 1
Sub-subnet 2
Sub-subnet 3
Sub-subnet 4
Sub-subnet 5
Sub-subnet 6
Sub-subnet 7
/30
/30
/30
/30
/30
/30
/30
/30
WAN
1&2
207.21.24.192
207.21.24.196
207.21.24.200
207.21.24.204
207.21.24.208
207.21.24.212
207.21.24.216
207.21.24.220
Free
Addresses
14
Addressing a Network Using VLSM
Through applying VLSM, the topology was able to be
addressed and still have two complete subnets available
for future growth.
Site B
Site A
207.21.24.192 /30
207.21.24.32 /27
25 users
207.21.24.196 /30
207.21.24.64 /27
25 users
Site C
207.21.24.160 /28
10 users
207.21.24.176 /28
8 users
15
Addressing a Network Using VLSM
Exercise 1
Your company has been assigned IP network 195.39.71.0 /24.
Given that headquarters (60 hosts) is connected to five branch
offices (12 hosts each) by a WAN link, and to an ISP (the ISP
owns the addresses on that link), determine an appropriate IP
addressing scheme.
Headquarters
ISP
60 users
Branch 1
12 users
Branch 2
12 users
Branch 3
12 users
Branch 4
12 users
Branch 5
12 users
16
Given the IP
address
195.39.71.0 /24,
subnet according
to the largest
subnet needed.
(Headquarters 60
hosts)
0
128
63
You would need to
borrow 2 bits or
/26. This would
give you 4
networks with 64
host addresses on
each subnet.
191
192
64
127
255
17
Playing it safe, we
will not use the
first subnet
(subnet 0).
0
64
We will start
addressing with
195.39.71.64 /26.
Headquarters
needs 60 hosts, so
we will assign
them .64 - .127.
128
192
Headquarters
60 hosts
26 bit mask or /26
(255.255.255.192)
18
The 5 Branch
offices only need
12 hosts each.
The next address
block available is
the .128 - .191
block (64
addresses). Here
we will apply
VLSM.
0
128
160
Branch 3
Branch 1
12 hosts
12 hosts
/28
/28
(255.255.255.240) (255.255.255.240)
144
176
Branch 4
Branch 2
12 hosts
12 hosts
/28
/28
(255.255.255.240) (255.255.255.240)
192
64
Headquarters
Using a /28 mask
will give us 16
hosts at each
location. This will
take care of 4 of
the Branch offices.
60 hosts
26 bit mask or /26
(255.255.255.192)
19
To obtain a block
for Branch 5, we
will need to subnet
the .192 - .255
block using a /28
mask.
0
128
160
Branch 3
Branch 1
12 hosts
12 hosts
/28
/28
(255.255.255.240) (255.255.255.240)
144
176
Branch 4
Branch 2
12 hosts
12 hosts
/28
/28
(255.255.255.240) (255.255.255.240)
192
64
Headquarters
60 hosts
26 bit mask or /26
224
Branch 5
12 hosts
/28
(255.255.255.240)
208
240
(255.255.255.192)
20
Now we need to
address the 5
WAN links that
connect to the
Branch offices.
These are pointto-point
connections and
only require 2
addresses.
Here we will use a
/30 mask to further
subnet the
subnets.
0
128
160
Branch 3
Branch 1
12 hosts
12 hosts
/28
/28
(255.255.255.240) (255.255.255.240)
144
176
Branch 4
Branch 2
12 hosts
12 hosts
/28
/28
(255.255.255.240) (255.255.255.240)
192
64
Headquarters
60 hosts
26 bit mask or /26
(255.255.255.192)
Branch 5
12 hosts
/28
(255.255.255.240)
208
WAN
1
212
WAN
2
216
224
232
228
236
WAN
5
240
WAN
3
220
WAN
4
21
Subnet 0 could be
used later if
needed for future
growth of HQ or
for more Branch
offices.
0
128
160
Branch 3
Branch 1
12 hosts
12 hosts
/28
/28
(255.255.255.240) (255.255.255.240)
144
176
Branch 4
Branch 2
12 hosts
12 hosts
/28
/28
(255.255.255.240) (255.255.255.240)
192
64
Headquarters
60 hosts
26 bit mask or /26
(255.255.255.192)
Branch 5
12 hosts
/28
(255.255.255.240)
208
WAN
1
212
WAN
2
216
224
232
228
236
WAN
5
240
WAN
3
220
WAN
4
22
Applying the Addresses to the Topology
Address
provided by ISP
195.39.71.64 /26
195.39.71.128 /28
195.39.71.144 /28
195.39.71.160 /28
195.39.71.176 /28
195.39.71.192 /28
23