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  2 32 binary combinations  4billion 3 classes of addresses in use (Classful) –Class A to –Class B to –Class C to additional classes –Class D to –Class E to

IP Classful Addresses = 126 Networks = 16,777,214 hosts 2 21 = 2,097,152 Networks = 254 Hosts 2 14 = 16,384 Networks = 65,534 Hosts

Address Notation An IP address can be written in two formats 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 address /21 The subnet mask is Class C address = = = = = = = = 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 /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 /24 is an all-zeros subnet /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 /26 [90/ ] via R /24 [120/4] via O /19 [110/229840] via A packet arrives destined to 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

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

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 Country: /19Ireland /19Scotland /19Wales /19England /19 not used /19not used /19not used /19not used

VLSM: Example NIC Number Town: 011/ /21London / /21Sheffield / /21Birmingham / /21not used Building 011/01/ /24 Owen / /24Howard / /24Harmer / /24Stoddart / /24For future use / /24For future use / /24For future use / /24For future use

VLSM: Example NIC Number Floors in the Harmer building 011/01/010./ /27Cannot be used / /271st Floor / /272nd Floor / /273rd Floor / /274th Floor / /27not used / /27not used / /27Cannot be used

VLSM: Example NIC Number Hosts on the 4th floor in the Harmer building 011/01/010./100/ /27The Floor / st host / nd host / rd 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 = = 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 Country: /19Ireland /19Scotland /19Wales /19England /19 Ireland connections /19Scotland connections /19Wales connections /19England connections

Example: England /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 2 11 possible networks / /30 Cannot be used / / / / / / /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