Chapter 6 vlsm and cidr
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Chapter 6 VLSM and CIDR. TECI 185 Routing Protocols and Concepts Jack Yon Western Colorado Community College [email protected] 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

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Chapter 6 VLSM and CIDR

TECI 185 Routing Protocols and Concepts

Jack Yon

Western Colorado Community College

[email protected]

Last Updated: 3/24/2009


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 IP Addressing

Classful Routing Protocols

Classless IP Addressing

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

  • 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?

  • 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 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 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 Protocol

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

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


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


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 = Route summarization

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


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


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


VLSM

VLSM in Action

VLSM and IP Addresses


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


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.


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

  • 10.2.0.0/16 subnet is also subnetted again with a /24 mask.

  • 10.3.0.0/16 subnet is subnetted again with the /28 mask.

  • 10.4.0.0/16 subnet is subnetted again with the /20 mask.


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

  • 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 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 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 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


VLSM

These subnets could be subnetted further!

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


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


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

255.255.255.240 or /28


VLSM 2

/30 – Gives 4 addresses

- 2 usable host addresses


VLSM 2 – Possible /30 options

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

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

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


CIDR

Route Summarization

Calculating Route Summarization


CIDR

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 (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 Summarization

  • 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 Summarization

  • 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 Summarization

  • Why is this static route a supernet?

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


More specific match?

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


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


Chapter 6 VLSM and CIDR

TECI 185 Routing Protocols and Concepts

Jack Yon

Western Colorado Community College

[email protected]

Last Updated: 3/24/2008


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