Integrated Network Services
Network Design
Almerindo Graziano
Menu
• 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
– Class B
– Class C
1.0.0.0
128.0.0.0
192.0.0.0
to
to
to
127.255.255.255
191.255.255.255
223.255.255.255
to
to
239.255.255.255
247.255.255.255
• 2 additional classes
– Class D
– Class E
224.0.0.0
240.0.0.0
IP Classful Addresses
27-2 = 126 Networks
224-2= 16,777,214 hosts
214 = 16,384 Networks
216-2 = 65,534 Hosts
221 = 2,097,152 Networks
28-2= 254 Hosts
Address Notation
• An IP address can be written in two formats
143.52.57.32 Dotted Decimal
10001111.00110100.00111001.00100000 Binary
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 address
200.128.48.0/21
The subnet mask is 255.255.248.0
Class C address
110010000.1000000.00110 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 10.1.1.1
R 192.168.32.0/24 [120/4] via 10.1.1.2
O 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?
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 Number
143.52.0.0
Country: 000
001
010
011
100
101
110
111
143.52.0.0/19
143.52.32.0/19
143.52.64.0/19
143.52.96.0/19
143.52.128.0/19
143.52.160.0/19
143.52.192.0/19
143.52.224.0/19
Ireland
Scotland
Wales
England
not used
not used
not used
not used
VLSM: Example
NIC Number
143.52.0.0
Town:
011/00
/01
/10
/11
Building
011/01/000
/001
/010
/011
/100
/101
/110
/111
143.52.96.0/21
143.52.104.0/21
143.52.112.0/21
143.52.120.0/21
London
Sheffield
Birmingham
not used
143.52.104.0/24
143.52.105.0/24
143.52.106.0/24
143.52.107.0/24
143.52.108.0/24
143.52.109.0/24
143.52.110.0/24
143.52.111.0/24
Owen
Howard
Harmer
Stoddart
For future use
For future use
For future use
For future use
VLSM: Example
NIC Number
143.52.0.0
Floors in the Harmer building
011/01/010./000
143.52.106.0/27
/001
143.52.106.32/27
/010
143.52.106.64/27
/011
143.52.106.96/27
/100
143.52.106.128/27
/101
143.52.106.160/27
/110
143.52.106.192/27
/111
143.52.106.224/27
Cannot be used
1st Floor
2nd Floor
3rd Floor
4th Floor
not used
not used
Cannot be used
VLSM: Example
NIC Number
143.52.0.0
Hosts on the 4th floor in the Harmer building
011/01/010./100/00000
143.52.106.128/27
/00001
143.52.106.129
/00010
143.52.106.130
/00011
143.52.106.131
.
.
.
.
.
.
/11111
143.52.106.159
The Floor
1st host
2nd host
3rd host
.
.
.
Broadcast
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 Number
143.52.0.0
Country: 000
001
010
011
100
101
110
111
143.52.0.0/19
143.52.32.0/19
143.52.64.0/19
143.52.96.0/19
143.52.128.0/19
143.52.160.0/19
143.52.192.0/19
143.52.224.0/19
Ireland
Scotland
Wales
England
Ireland connections
Scotland connections
Wales connections
England connections
Example: England
143.52.224.0/19 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
00000.000001
00000.000010
00000.000011
00000.000100
00000.000101
.
.
.
.
11111.111110
11111.111111
143.52.224.0/30
Cannot be used
143.52.224.4/30
143.52.224.8/30
143.52.224.12/30
143.52.224.16/30
143.52.224.20/30
.
.
.
.
143.52.255.248/30
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