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Chapter 6 VLSM and CIDR. TECI 185 Routing Protocols and Concepts Jack Yon Western Colorado Community College jyon@mesastate.edu Last Updated: 3/24/2009. Topics. Classful and Classless Addressing Classful IP Addressing Classful Routing Protocols Classless IP Addressing

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Chapter 6 vlsm and cidr

Chapter 6 VLSM and CIDR

TECI 185 Routing Protocols and Concepts

Jack Yon

Western Colorado Community College

jyon@mesastate.edu

Last Updated: 3/24/2009


Topics
Topics

  • Classful and Classless Addressing

    • Classful IP Addressing

    • Classful Routing Protocols

    • Classless IP Addressing

    • Classless Routing Protocols

  • VLSM

    • VLSM in Action

    • VLSM and IP Addresses

  • CIDR

    • Route Summarization

    • Calculating Route Summarization


Classful and classless addressing

Classful and Classless Addressing

Classful IP Addressing

Classful Routing Protocols

Classless IP Addressing

Classless Routing Protocols


Classful and classless routing protocols
Classful and Classless Routing Protocols

  • Routing protocols:

    • classful or classless.

  • This is a result of the evolution from classful to classless IPv4 addressing.

  • As networks began to use classless addressing, classless routing protocols had to be modified or developed to include the subnet mask in the routing update.


Classful ip addressing
Classful IP Addressing

  • As of January 2007, there were over 433 million hosts on the Internet.

  • IPv4 32-bit address space would now be exhausted if it were not for?

    • VLSM - 1993 (RFC 1519)

    • CIDR - 1993 (RFC 1519)

    • Network Address Translation (NAT) - 1994 (RFC 1631)

    • Private addressing- 1996 (RFC 1918)


High order bits
High-Order Bits?

  • Only these three choices - No medium sized networks .

  • How did they actually come up with these ranges?

  • How can a device such as a router quickly determine the subnet mask of the IP address?

    • By examining the first few bits of the address.


Classful routing protocol
Classful Routing Protocol

Classful Routing Protocols

  • Is the subnet mask included in the routing update?

    • No

  • How does the router determine the mask?

    • Value of the first octet (first 3 bits of the address) or Interface Mask


Classful routing protocol1
Classful Routing Protocol

R2 applies s0/0/0’s /24 subnet mask (same major network)

R1 sends a subnet address out s0/0/0 (same major network)


Classful routing protocol2
Classful Routing Protocol

R2 sends a summarized route out s0/0/1 (different major network)

R3 applies the default /16 subnet mask (different major network)


Moving toward classless addressing
Moving TowardClassless Addressing

  • By 1992, IETF had serious concerns about:

    • The exponential growth of the Internet and Internet routing tables.

    • Eventual exhaustion of 32-bit IPv4 address space.

  • 1993, IETF introduced classless interdomain routing (CIDR) (RFC 1517).

    • More efficient use of IPv4 address space

    • Prefix aggregation, which reduced the size of routing tables


Chapter 6 vlsm and cidr

CLASS A

ISPs no longer restricted to three classes. Can now allocate a large range of network addresses based on customer requirements

CLASS B

11111111.00000000.00000000.00000000 /8 (255.0.0.0) 16,777,216 host addresses

11111111.10000000.00000000.00000000 /9 (255.128.0.0) 8,388,608 host addresses

11111111.11000000.00000000.00000000 /10 (255.192.0.0) 4,194,304 host addresses

11111111.11100000.00000000.00000000 /11 (255.224.0.0) 2,097,152 host addresses

11111111.11110000.00000000.00000000 /12 (255.240.0.0) 1,048,576 host addresses

11111111.11111000.00000000.00000000 /13 (255.248.0.0) 524,288 host addresses

11111111.11111100.00000000.00000000 /14 (255.252.0.0) 262,144 host addresses

11111111.11111110.00000000.00000000 /15 (255.254.0.0) 131,072 host addresses

11111111.11111111.00000000.00000000 /16 (255.255.0.0) 65,536 host addresses

11111111.11111111.10000000.00000000 /17 (255.255.128.0) 32,768 host addresses

11111111.11111111.11000000.00000000 /18 (255.255.192.0) 16,384 host addresses

11111111.11111111.11100000.00000000 /19 (255.255.224.0) 8,192 host addresses

11111111.11111111.11110000.00000000 /20 (255.255.240.0) 4,096 host addresses

11111111.11111111.11111000.00000000 /21 (255.255.248.0) 2,048 host addresses

11111111.11111111.11111100.00000000 /22 (255.255.252.0) 1,024 host addresses

11111111.11111111.11111110.00000000 /23 (255.255.254.0) 512 host addresses

11111111.11111111.11111111.00000000 /24 (255.255.255.0) 256 host addresses

11111111.11111111.11111111.10000000 /25 (255.255.255.128) 128 host addresses

11111111.11111111.11111111.11000000 /26 (255.255.255.192) 64 host addresses

11111111.11111111.11111111.11100000 /27 (255.255.255.224) 32 host addresses

11111111.11111111.11111111.11110000 /28 (255.255.255.240) 16 host addresses

11111111.11111111.11111111.11111000 /29 (255.255.255.248) 8 host addresses

11111111.11111111.11111111.11111100 /30 (255.255.255.252) 4 host addresses

11111111.11111111.11111111.11111110 /31 (255.255.255.254) 2 host addresses

11111111.11111111.11111111.11111111 /32 (255.255.255.255) “Host Route”

CLASS C


Cidr and route summarization
CIDR and Route Summarization

  • CIDR = Route summarization

  • A supernet summarizes multiple network addresses with a mask less than the classful mask.


Cidr and route summarization1
CIDR and Route Summarization

  • 192.168.0.0/23, 192.168.2.0/23, 192.168.4.0/22, and 192.168.8.0/21 are all subnets of 192.168.0.0/20


Cidr and route summarization2
CIDR and Route Summarization

  • Propagating VLSM and supernet routes requires a classless routing protocol, because the subnet mask can no longer be determined by the value of the first octet.


Classless routing protocol
Classless Routing Protocol

  • Classless routing protocols include the subnet mask with the network address in their routing updates.


Classless routing protocol1
Classless Routing Protocol

  • 172.16.0.0/16, 172.17.0.0/16, 172.18.0.0/16, and 172.19.0.0/16 summarized as 172.16.0.0/14.

  • What is this called? (Subnet mask is less than the classful default mask.)

    • Supernet

    • /14 (255.252.0.0) subnet mask is included in the routing update.

/14


Chapter 6 vlsm and cidr

VLSM

VLSM in Action

VLSM and IP Addresses


Chapter 6 vlsm and cidr
VLSM

  • The network 10.0.0.0/8 has been subnetted using the subnet mask of /16, which gives the potential of 256 subnets:

    10.0.0.0/16

    10.1.0.0/16

    10.2.0.0/16

    .

    .

    .

    10.255.0.0/16


Chapter 6 vlsm and cidr
VLSM

  • Any of these /16 subnets can be subnetted further.

  • For example the 10.1.0.0/16 subnet is subnetted again using the /24 mask.


Chapter 6 vlsm and cidr


Chapter 6 vlsm and cidr

A

10.1.4.10/24

  • Individual host addresses are assigned from the addresses of “sub-subnets.”

  • What would be a valid Host IP address for Host A?


Vlsm a different way to look at it
VLSM: A different way to look at it

  • Subnet 10.0.0.0/8 into /16 subnets.

  • Subnet 10.1.0.0/16 into /24 subnets.


Vlsm a different way to look at it1
VLSM: A different way to look at it

  • Subnet 10.2.0.0/16 into /24 subnets.

    • Subnets ranging from 10.2.0.0/24 to 10.2.255.0/24


Vlsm a different way to look at it2
VLSM: A different way to look at it

  • Subnet 10.3.0.0/16 is further subnetted with a /28 mask

    • 14 host addresses per subnet.

    • Subnets ranging from 10.3.0.0/28 to 10.3.255.240/28.


Vlsm a different way to look at it3
VLSM: A different way to look at it

  • Subnet 10.4.0.0/16 subnetted with a /20 mask

    • 4094 host addresses per subnet

    • subnets ranging from 10.4.0.0/20 to 10.4.240.0/20


Chapter 6 vlsm and cidr
VLSM

These subnets could be subnetted further!

All other /16 subnets are still available for use as /16 networks or to be subnetted.


Chapter 6 vlsm and cidr

Hosts are assigned an IP address and mask from a specific subnet.

VLSM

What are the valid host IP Addresses?

10.2.1.55/24

10.2.5.55/24

All other /16 subnets are still available for use as /16 networks or to be subnetted.

10.255.0.5/16

10.4.0.55/20


Chapter 6 vlsm and cidr

Host can only be a member of the subnet. Host can NOT be a member of the network that was subnetted.

VLSM

Are these valid host IP Addresses?

YES!

10.2.1.55/24

10.2.0.55/16

NO!

All other /16 subnets are still available for use as /16 networks or to be subnetted.


Vlsm 1
VLSM 1 member of the network that was subnetted.

255.255.255.240 or /28


Vlsm 2
VLSM 2 member of the network that was subnetted.

/30 – Gives 4 addresses

- 2 usable host addresses


Vlsm 2 possible 30 options
VLSM 2 – Possible /30 options member of the network that was subnetted.

Conflicts

Existing /27 Networks

1286432168421

.64 0 1 0 0 0 0 0 0

.96 0 1 1 0 0 0 0 0

.128 1 0 0 0 0 0 0 0

---------------------------------------

.113 0 1 1 1 0 0 0 1

.145 1 0 0 1 0 0 0 1

.193 1 1 00 0 0 0 1

Conflict

Conflict

/30 Choices

Answer


Vlsm 2 our new vslm subnet
VLSM 2 – Our new VSLM Subnet member of the network that was subnetted.

Existing /27 Networks

1286432168421

.64 0 1 0 0 0 0 0 0

.96 0 1 1 0 0 0 0 0

.128 1 0 0 0 0 0 0 0

----------------------------------------------

.192 1 1 00 0 0 0 0 (Net)

.193 1 1 00 0 0 0 1 (1st hst)

.194 1 1 00 0 0 1 0 (2nd hst)

.195 1 1 00 0 0 1 1 (Bcast)

.192 Network


Vlsm 2 other vlsm subnets
VLSM 2 – Other VLSM Subnets member of the network that was subnetted.

Existing /27 Networks

.192 Network

1286432168421

.64 0 1 0 0 0 0 0 0

.96 0 1 1 0 0 0 0 0

.128 1 0 0 0 0 0 0 0

---------------------------------------

.192 1 1 00 0 0 0 0

.196 1 1 00 0 1 0 0

.200 1 1 00 1 0 0 0

.204 1 1 00 1 1 0 0

.208 1 1 01 0 0 0 0

.212 1 1 01 0 1 0 0

.216 1 1 01 1 0 0 0

.220 1 1 01 1 1 0 0

Other /30 Networks


Chapter 6 vlsm and cidr

CIDR member of the network that was subnetted.

Route Summarization

Calculating Route Summarization


Chapter 6 vlsm and cidr
CIDR member of the network that was subnetted.

CIDR Report: www.cidr-report.org

  • CIDR allows routing protocols to summarize multiple networks, a block of addresses, as a single route.

  • An example is 172.16.1.0/24.


Route summarization
Route Summarization member of the network that was subnetted.

  • Route summarization (route aggregation) - Process of advertising a contiguous set of addresses as a single address with a less-specific, shorter subnet mask.

  • Remember that CIDR is a form of route summarization and is synonymous with the term…?

    • Supernetting.


Route summarization1
Route Summarization member of the network that was subnetted.

  • CIDR ignores the limitation of classful boundaries and allows summarization with masks that are less than that of the default classful mask.

  • What type of routing protocols can propagate (distribute) supernets?

    • Classless routing protocols

  • Why?

    • Classless routing protocols include both the network address and the mask in the routing update.

  • Why can’t a classful routing protocol propagate supernets?

    • Classful routing protocols cannot include supernets in their routing updates because they cannot apply a mask less than the default classful mask.


Route summarization2
Route Summarization member of the network that was subnetted.

  • For example, RIPv1 will summarize 172.30.0.0/24 subnets (172,30.1.0/24, 172.30.2.0/24 and 172.30.3.0/24) as 172.30.0.0.

  • R3 applies the /8 mask (classful routing protocol)


Route summarization3
Route Summarization member of the network that was subnetted.

  • Why is this static route a supernet?

    • The /13 mask is less than the default Class B /16.


More specific match
More specific match? member of the network that was subnetted.

Different example from book.

172.16.0.0/16

172.16.10.0/24

S0/0/0

S0/0/1

  • Could a router have both a specific route entry and a summary route entry covering the same network.

    • Yes

  • What if a packet with the destination IP address 172.16.10.10 entered this router? Where would it be forwarded and why?

    • The packet has a more specific (longer) match with 172.16.10.0/24, so S0/0/1 would be used to forward this packet.

    • A minimum of 24 bits match between the IP address and the route.

  • What is a packets with the destination IP address 172.16.20.10 entered this router? Where would it be forwarded and why?

    • The packet only has a match with the less specific172.16.10.0/24, so S0/0/1 would be used to forward this packet

    • A minimum of 16 bits match between the IP address and the route.


Calculating route summarization
Calculating Route Summarization member of the network that was subnetted.

  • Calculating route summaries and supernets is identical to the process that you already learned in Chapter 2.


Topics1
Topics member of the network that was subnetted.

  • Classful and Classless Addressing

    • Classful IP Addressing

    • Classful Routing Protocols

    • Classless IP Addressing

    • Classless Routing Protocols

  • VLSM

    • VLSM in Action

    • VLSM and IP Addresses

  • CIDR

    • Route Summarization

    • Calculating Route Summarization


Chapter 6 vlsm and cidr1

Chapter 6 VLSM and CIDR member of the network that was subnetted.

TECI 185 Routing Protocols and Concepts

Jack Yon

Western Colorado Community College

jyon@mesastate.edu

Last Updated: 3/24/2008