Integrated Network Services

Network Design

Almerindo Graziano

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• Issues in Network Design

• Designing an addressing scheme with IP4– Classful vs Classless addressing

• How to choose and addressing scheme

• How to choose an adequate routing protocol

• Examples

• Recap

Designing a new Network

• How many networks do currently exist?

• How many will exist in the next 3-12 months?

• How many hosts on each network?

• How many will exist in the next 3-12 months?– Is the number constant or does it vary with the

network?• Does the logical topology reflect the physical

topology?

Designing a new Network

• Do you have a client/server environment?– Where are the server located?– Do you need access every network from every network?

• Any security issues?• Any mobile users?• Do you need Internet access?

– Do you have your own NIC address or are you connected via an ISP

• Which network protocol will you be adopting?

Designing an Addressing Scheme with IP

• Current version is IP4• 32-bit long 232 binary combinations 4billion• 3 classes of addresses in use (Classful)

– Class A 1.0.0.0 to 127.255.255.255– Class B 128.0.0.0 to 191.255.255.255– Class C 192.0.0.0 to 223.255.255.255

• 2 additional classes– Class D 224.0.0.0 to 239.255.255.255– Class E 240.0.0.0 to 247.255.255.255

IP Classful Addresses

27-2 = 126 Networks224-2= 16,777,214 hosts

221 = 2,097,152 Networks28-2= 254 Hosts

214 = 16,384 Networks216-2 = 65,534 Hosts

Address Notation

• An IP address can be written in two formats

143.52.57.32

10001111.00110100.00111001.00100000 Binary

Dotted Decimal

IP4 addresses

• Addresses allocated by IANA– APNIC (Asia-Pacific)– ARIN (American Registry for Internet Numbers)– RIPE (Reseu IP Europeens)

Limitations

• Lack of support for medium-size organization– Example: 300 hosts needed a Class B address– Rapid depletion of class B addresses

• Large routing tables– No way to summarize large number of addresses

• Routers DO need to know how to reach each network

Next Step: Subnetting

• How can we use our network address efficiently?– RFC 950 (subnetting) : use some of the host bits

to represent a network– More freedom and flexibility within an

organization (no need to speak to IANA) • Increased complexity within the organization

– More stable routing tables (reduced flapping)– Still large routing tables

CIDR

• What do we do if we need 1600 addresses?

• CIDR (Classless InterDomain Routing)

• Why Classless– The network is not identified by the Class (first

octect rule)– The network is identified by a subnet mask– The subnet mask identifies the boundary

between the network part and the host part

CIDR

• A company needs 1600 address

• In the old days:– 1600/254 = 6.29 7 Class C addresses– 7 Class C addresses 7 new entries on the Internet– Alternatively use 1 Class B address (What a waste!!)

• Using CIDR– IANA releases 8 Class C addresses– 8 Class C addresses 1 new entry on the Internet

CIDR example• RIPE will provide this address200.128.48.0/21 The subnet mask is 255.255.248.0

Class C address110010000.1000000.00110 000 .00000000 = 200.128.48.0

001 .00000000 = 200.128.49.0 010 .00000000 = 200.128.50.0 011 .00000000 = 200.128.51.0 100 .00000000 = 200.128.52.0 101 .00000000 = 200.128.53.0 110 .00000000 = 200.128.54.0 111 .00000000 = 200.128.55.0

• 8 Class C addresses are allocated to the company• Only 7 are used• Only one new entry is advertised to the internet

CIDR Example

CIDR

• Summarization

• Smaller routing tables Less Memory

• Smaller routing tables Less CPU

• Smaller updates Less traffic

• More stable routing tables (not always good)– Unecessary traffic can traverse the network for a

while

Classless Routing• What happens if we decide to use different subnet

mask? (Networks are different!!)– First routing protocols didn’t send the subnet mask

(RIP1)• The mask was configured locally• No masks are associated with entries in the routing table• It is assumed that the same mask is used on the major network• The router has no way of knowing how many bits are used for

the host

– SHU uses 143.52.0.0/24

Classful Routing• Classful lookup: a packet arrives at in interface 1) The network portion of the destination address is

read

2) If the there is no match for the major networks (A, B, C) the packet is dropped (ICMP)

3) A match is found for a major network - All the subnets listed for that network are

examined - A match is found and the packet is routed - A match is not found and the packet is dropped

• The destination address is interpreted according to the subnet masks locally configured on the router

Classful and Classless Routing Protocols

• Classless routing protocols send the subnet mask in their route advertisements

• Benefits– All-zeros and All-ones subnets can be used 143.52.0.0/24 is an all-zeros subnet 143.52.0.0/16 is the major network number– It is possible to use VLSM (Variable Length Subnet Mask)– It is possible to summarize a group of major networks with

one aggregate address

• A classful routing protocol doesn’t advertise routes between interfaces whose mask don’t match

Classful and Classless Routing Protocols

• Examples of Classful routing protocols– RIP1– IGRP– EGP

• Examples of Classless routing protocols– RIP2– OSPF– EIGRP– BGP4

Classless Routing

• Classless lookup: a packet arrives at an interface• The router doesn’t pay attention to the class of the

destination address• The router performs a bit-by-bit comparison

between the destination address and all the routing entries

Classless Routing: Example

• A router has the following routing table

D 192.168.32.0/26 [90/25789217] via 10.1.1.1R 192.168.32.0/24 [120/4] via 10.1.1.2O 192.168.32.0/19 [110/229840] via 10.1.1.3

• A packet arrives destined to 192.168.32.1• Which network will it be forwarded to?

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VLSM

• Using a single subnet mask has disadvantages– Inefficient use of address space– No summarization possible

• Variable Length Subnet Mask (VLSM)– Recursive division of an address space– Allows route aggregation– Efficient use of the address space– Requires new-generation protocols (RIP2, OSPF,

EIGRP etc.)

Rules for VLSM

• A subnet can be used for– Address host– Further subnetting

• It is possible to use the all-zeros and all-ones subnets– The rule has to be obeyed only once!– Usually use the least significant bits

VLSM: Example

• A organization is spread across – England, Ireland, Scotland and Wales– In each of these countries the organization has a

presence in no more than 3 towns– In each town the company has max 4 buildings (the

number may increase)– No building has more than 4 floors– No floor has more than 20 hosts

• The organization is assigned 143.52.0.0

VLSM: Example

• Step 1– Identify the number of bits we need for

identifying Countries, Towns, Buildings etc.– We can play with 16 bits Host Floor Building Town Country

00110 010.100 00011

VLSM: Example

• Step 2– Identify where we will apply the subnet rule

• We will use the floor bits

• Step 3– Identify addresses for WAN connections

• Example: borrow a subnet from available ones:– 4 country Subnets – 1 town subnet per country– 2 floors per building

• Document the choice made

VLSM: Example

NIC Number143.52.0.0

Country: 000 143.52.0.0/19 Ireland 001 143.52.32.0/19 Scotland 010 143.52.64.0/19 Wales 011 143.52.96.0/19 England 100 143.52.128.0/19 not used 101 143.52.160.0/19 not used 110 143.52.192.0/19 not used 111 143.52.224.0/19 not used

VLSM: ExampleNIC Number143.52.0.0

Town:011/00 143.52.96.0/21 London /01 143.52.104.0/21 Sheffield /10 143.52.112.0/21 Birmingham /11 143.52.120.0/21 not used

Building011/01/000 143.52.104.0/24 Owen

/001 143.52.105.0/24 Howard /010 143.52.106.0/24 Harmer /011 143.52.107.0/24 Stoddart /100 143.52.108.0/24 For future use /101 143.52.109.0/24 For future use /110 143.52.110.0/24 For future use /111 143.52.111.0/24 For future use

VLSM: Example

NIC Number143.52.0.0

Floors in the Harmer building011/01/010./000 143.52.106.0/27 Cannot be used /001 143.52.106.32/27 1st Floor

/010 143.52.106.64/27 2nd Floor /011 143.52.106.96/27 3rd Floor /100 143.52.106.128/27 4th Floor /101 143.52.106.160/27 not used /110 143.52.106.192/27 not used /111 143.52.106.224/27 Cannot be used

VLSM: Example

NIC Number143.52.0.0

Hosts on the 4th floor in the Harmer building011/01/010./100/00000 143.52.106.128/27 The Floor /00001 143.52.106.129 1st host /00010 143.52.106.130 2nd host /00011 143.52.106.131 3rd host

. . . . . . . . .

/11111 143.52.106.159Broadcast

Interconnection Requirements• In each town the buildings are interconnected in a mesh via Frame Relay

with dial-up connections for back up. – Each building needs 3(buildings) x 2 = 6 addresses

• Potentially 7( buildings) x 2 = 14 addresses (if we have 8 buildings)– Each town needs 4(buildings) x 6 = 24 addresses

• Potentially 8(buildings) x 14 = 112 addresses (if we have 8 buildings)

• Each town has a central site, located within one of the buildings, which connects to the other towns via Frame_Relay with dial-up connections for back up– Each town needs at least 2(towns) x 2 = 4 addresses for serial connections– Each country needs a total of 3(towns) x 4 = 12 addresses for serial connections

• Each town is connected to the rest of the company via an ISP, using VPN technology. Some resilience is also needed– Each country needs at least 3 x 2 = 6 addresses

Total addresses per country = 112 + 12 + 6 = 130

Summary of requirements

• Within each town we need:– 112 addresses between buildings (64 networks)

• For each country we need:– 12 addresses for connections between towns (6 networks)

– 6 addresses for connections between a town and the ISP (3 networks)

• For each country we can use a different subnet of the ones available in the country subnets

VLSM: Example

NIC Number143.52.0.0

Country: 000 143.52.0.0/19 Ireland 001 143.52.32.0/19 Scotland 010 143.52.64.0/19 Wales 011 143.52.96.0/19 England 100 143.52.128.0/19 Ireland connections 101 143.52.160.0/19 Scotland connections 110 143.52.192.0/19 Wales connections 111 143.52.224.0/19 England connections

Example: England143.52.224.0/19 can be further subnetted for serial connections

• 13 bits to play with• Each serial connection will have a subnet mask of /30• We have 211 possible networks

143.52.111/00000.000000 143.52.224.0/30 Cannot be used 00000.000001 143.52.224.4/30 00000.000010 143.52.224.8/30 00000.000011 143.52.224.12/30 00000.000100 143.52.224.16/30 00000.000101 143.52.224.20/30

. . . .

. . . . 11111.111110 143.52.255.248/30 11111.111111 143.52.255.252/30 Cannot be used

Recap• Issues in Network design• Designing an Addressing scheme with IP4• Subnetting• CIDR• Classful vs Classless• VLSM• Examples• References:

Understanding IP Addressing: Everything You Ever Wanted To Know. 3COM whitepaper• http://www.3com.com/other/pdfs/infra/corpinfo/en_US/501302.pdf