ccna variable length subnet mask (vlsm). 2 what is vlsm? a variable length subnet mask (vlsm) is a...
TRANSCRIPT
CCNA
Variable Length Subnet Mask
(VLSM)
2
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).
3
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.
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
207.21.24.0 /24
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?
Site A Site B Site C
25 users 25 users 10 users 8 users
5
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
25 users 25 users 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.
6
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).
Site A Site B Site C
25 users 25 users 10 users 8 users
Subnet # Subnet AddressBits Masked
0 207.21.24.0 /271 207.21.24.32 /272 207.21.24.64 /273 207.21.24.96 /274 207.21.24.128 /275 207.21.24.160 /276 207.21.24.192 /277 207.21.24.224 /27
207.21.24.0
7
Subnetting in a Box
0
255
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. 256 addresses
8
Subnetting in a Box
0
255
128
127
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.
The subnet mask would be 255.255.255.128.
128 addresses 128 addresses
9
Subnetting in a Box
0
255
128
127
64 192
63 191
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.
64 addresses 64 addresses
64 addresses 64 addresses
10
Subnetting in a Box
0
255
128
127
64 192
63 191
If we were to borrow 3 bits, it would break each of these 4 networks in half again. This would give us 8 networks, each with 32 addresses.
The subnet mask would now be 255.255.255.224.
32addresses
32addresses
31
32
32addresses
32addresses
95
96
32addresses
32addresses
159
160
32addresses
32addresses
223
224
11
Subnetting in a Box
0
255
128
127
64 192
63 191
If we were to borrow 4 bits, it would break each of these 8 networks in half again. This would give us 16 networks, each with 16 addresses.
The subnet mask would now be 255.255.255.240.
31
32
95
96
159
160
223
224
16addresses
16addresses
16addresses
16addresses
16addresses
16addresses
16addresses
16addresses
16addresses
16addresses
16addresses
16addresses
16addresses
16addresses
16addresses
16addresses
16
15
48
47
144
143
176
175
80
79
112
111
208
207
240
239
12
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)
Classless Routing Protocols(do share subnet info)
RIP v1 RIP v2IGRP EIGRP
OSPFIS-IS
13
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)
Site A Site B Site C
25 users 25 users 10 users 8 users
Subnet # Subnet AddressBits Masked
0 207.21.24.0 /271 207.21.24.32 /272 207.21.24.64 /273 207.21.24.96 /274 207.21.24.128 /275 207.21.24.160 /276 207.21.24.192 /277 207.21.24.224 /27
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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 A Site B Site C
25 users 25 users 10 users 8 users
Subnet # Subnet Address
0 207.21.24.0 /271 207.21.24.32 /272 207.21.24.64 /273 207.21.24.96 /274 207.21.24.128 /275 207.21.24.160 /276 207.21.24.192 /277 207.21.24.224 /27
Site A
Site B & CWAN links
Free Addresses
Sub-subnet 0 207.21.24.192 /30Sub-subnet 1 207.21.24.196 /30Sub-subnet 2 207.21.24.200 /30Sub-subnet 3 207.21.24.204 /30Sub-subnet 4 207.21.24.208 /30Sub-subnet 5 207.21.24.212 /30Sub-subnet 6 207.21.24.216 /30Sub-subnet 7 207.21.24.220 /30
Free Addresses
WAN 1 & 2
Sub-subnet 0 207.21.24.160 /28Sub-subnet 1 207.21.24.176 /28
Site BSite C
15
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 A Site B Site C
25 users 25 users 10 users 8 users
207.21.24.32 /27 207.21.24.64 /27 207.21.24.160 /28
207.21.24.176 /28
207.21.24.192 /30
207.21.24.196 /30
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Addressing a Network Using VLSMExercise 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
Branch 1
60 users
12 users 12 users 12 users 12 users 12 usersBranch 2 Branch 3 Branch 4 Branch 5
ISP
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Given the IP address 195.39.71.0 /24, subnet according to the largest subnet needed. (Headquarters 60 hosts)
0
255
128
127
64 192
63 191
You would need to borrow 2 bits or /26. This would give you 4 networks with 64 host addresses on each subnet.
18
Playing it safe, we will not use the first subnet (subnet 0).
0
64
128
192We will start addressing with 195.39.71.64 /26. Headquarters needs 60 hosts, so we will assign them .64 - .127.
Headquarters
60 hosts
26 bit mask or /26
(255.255.255.192)
19
The 5 Branch offices only need 12 hosts each.
0
64
128
192
The next address block available is the .128 - .191 block (64 addresses). Here we will apply VLSM.
Headquarters
60 hosts
26 bit mask or /26
(255.255.255.192)
Using a /28 mask will give us 16 hosts at each location. This will take care of 4 of the Branch offices.
160
144 176
Branch 112 hosts
/28(255.255.255.240)
Branch 212 hosts
/28(255.255.255.240)
Branch 312 hosts
/28(255.255.255.240)
Branch 412 hosts
/28(255.255.255.240)
20
To obtain a block for Branch 5, we will need to subnet the .192 - .255 block using a /28 mask.
0
64
128
192
Headquarters
60 hosts
26 bit mask or /26
(255.255.255.192)
160
144 176
Branch 112 hosts
/28(255.255.255.240)
Branch 212 hosts
/28(255.255.255.240)
Branch 312 hosts
/28(255.255.255.240)
Branch 412 hosts
/28(255.255.255.240)
224
208 240
Branch 512 hosts
/28(255.255.255.240)
21
Now we need to address the 5 WAN links that connect to the Branch offices. These are point-to-point connections and only require 2 addresses.
0
64
128
192
Here we will use a /30 mask to further subnet the subnets.
Headquarters
60 hosts
26 bit mask or /26
(255.255.255.192)
160
144 176
Branch 112 hosts
/28(255.255.255.240)
Branch 212 hosts
/28(255.255.255.240)
Branch 312 hosts
/28(255.255.255.240)
Branch 412 hosts
/28(255.255.255.240)
224
208 240
Branch 512 hosts
/28(255.255.255.240)
216
212 220
232
228 236
WAN 1
WAN 2
WAN 3
WAN 4
WAN 5
22
Subnet 0 could be used later if needed for future growth of HQ or for more Branch offices.
0
64
128
192
Headquarters
60 hosts
26 bit mask or /26
(255.255.255.192)
160
144 176
Branch 112 hosts
/28(255.255.255.240)
Branch 212 hosts
/28(255.255.255.240)
Branch 312 hosts
/28(255.255.255.240)
Branch 412 hosts
/28(255.255.255.240)
224
208 240
Branch 512 hosts
/28(255.255.255.240)
216
212 220
232
228 236
WAN 1
WAN 2
WAN 3
WAN 4
WAN 5
23
Address provided by ISP195.39.71.64 /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
195.39.71.208 /3019
5.39
.71.
212
/30
195.
39.7
1.21
6 /3
0
195.39.71.220 /30
195.39.71.224 /30
Applying the Addresses to the Topology