CCNA v3.0 Retooling

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).

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.

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

207.21.24.0 /24

25 users 25 users 10 users How many network addresses are needed?

8 users How many hosts are needed for the largest LAN?

How many bits need to be borrowed to address this network?

Addressing a Network with Standard Subnetting

Site B • Site A has two Ethernet networks • Site B had one Ethernet network • Site C had one Ethernet network Site A 25 users 25 users 10 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. Site C 8 users 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 Site A

207.21.24.0

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.

The subnet mask would be 255.255.255.128.

0 128 addresses 128 128 addresses 127 255 8

Subnetting in a Box

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.

0 64 64 addresses 64 addresses 128 63 192 64 addresses 64 addresses 191 127 255 9

Subnetting in a Box

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.

0 64 32 addresses 32 addresses 31 32 addresses 96 32 32 63 addresses 128 addresses 192 32 32 159 addresses 160 addresses 224 32 32 191 addresses 95 127 223 255 10

Subnetting in a Box

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.

0 16 addresses 15 32 16 addresses 47 128 16 addresses 143 160 16 addresses 175 16 16 addresses 31 48 16 addresses 63 144 16 addresses 159 176 16 addresses 191 64 16 addresses 79 96 16 addresses 111 192 16 addresses 207 224 16 addresses 239 80 16 addresses 95 112 16 addresses 127 208 16 addresses 223 240 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

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 10 users Site C 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 A Site B Site C Site A

Free Addresses

Site B & C WAN links Subnet # Subnet Address 5 6 7 0 1 2 3 4 207.21.24.0 /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 Sub-subnet 0 207.21.24.160 /28 Sub-subnet 1 207.21.24.176 /28 Sub-subnet 0 207.21.24.192 /30 Sub-subnet 1 207.21.24.196 /30 Sub-subnet 2 207.21.24.200 /30 Sub-subnet 3 207.21.24.204 /30 Sub-subnet 4 207.21.24.208 /30 Sub-subnet 5 207.21.24.212 /30 Sub-subnet 6 207.21.24.216 /30 Sub-subnet 7 207.21.24.220 /30 8 users Site B Site C WAN 1 & 2

Free Addresses

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 C Site A 207.21.24.192 /30 207.21.24.196 /30 207.21.24.32 /27 25 users 207.21.24.64 /27 25 users 10 users 207.21.24.160 /28 8 users 207.21.24.176 /28 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.

60 users Headquarters ISP 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 You would need to borrow 2 bits or /26. This would give you 4 networks with 64 host addresses on each subnet.

64 128 63 192 191 127 255 17

Playing it safe, we will not use the first subnet (subnet 0). 0 128 We will start addressing with 195.39.71.64 /26. Headquarters needs 60 hosts, so we will assign them .64 - .127.

64 Headquarters 60 hosts 26 bit mask or /26 (255.255.255.192) 192 18

The 5 Branch offices only need 12 hosts each. 0 The next address block available is the .128 - .191 block (64 addresses). Here we will apply VLSM.

64 Using a /28 mask will give us 16 hosts at each location. This will take care of 4 of the Branch offices.

Headquarters 60 hosts 26 bit mask or /26 (255.255.255.192) 128 Branch 1 12 hosts /28 (255.255.255.240) 160 Branch 3 12 hosts /28 (255.255.255.240) 144 Branch 2 12 hosts /28 (255.255.255.240) 176 Branch 4 12 hosts /28 (255.255.255.240) 192 19

To obtain a block for Branch 5, we will need to subnet the .192 - .255 block using a /28 mask.

0 64 Headquarters 60 hosts 26 bit mask or /26 (255.255.255.192) 128 Branch 1 12 hosts /28 (255.255.255.240) 160 Branch 3 12 hosts /28 (255.255.255.240) 144 Branch 2 12 hosts /28 (255.255.255.240) 176 Branch 4 12 hosts /28 (255.255.255.240) 192 Branch 5 12 hosts /28 (255.255.255.240) 224 208 240 20

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. Here we will use a /30 mask to further subnet the subnets.

Subnet 0 could be used later if needed for future growth of HQ or for more Branch offices.

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

CCNA v3.0 Retooling

Transcript CCNA v3.0 Retooling

Variable Length Subnet Mask (VLSM)

What is VLSM?

Subnetting vs. VLSM

Addressing a Network with Standard Subnetting

Addressing a Network with Standard Subnetting

Addressing a Network with Standard Subnetting

Addressing a Network Using VLSM

Addressing a Network Using VLSM

Addressing a Network Using VLSM

Addressing a Network Using VLSM

Addressing a Network Using VLSM Exercise 1

Applying the Addresses to the Topology

Directory