IP Addressing and Introduction to IP routing

1
IP Addressing and Introduction to IP routing

• Avgust Jauk ltjauk_at_arnes.sigt
• ARNES

Budapest, August 99
2
Agenda

• Internet topology
• Introduction to addressing
• Idea of routing
• Special address conventions
• Classfull addressing
• Classless addressing

3
Internet topology

• Internet - Network of Networks
• Networks
• Based on different technology
• Large or small
• Fast or slow
• Variety of connected nodes
• Routers (Gateways)
• Protocols

4
Internet topology
5
Routers

• Packet handling
• Packet forwarding
• Routing information processing
• Management
• Miscellaneous functions

6
Internet protocol stack
7
Internet protocol dependencies

Users
Hardware
8
Layering in the Internet
Host A
Host B
Identical message
Application
Application
Identical packet
Transport
Transport
Gateway G
Internet
Internet
Internet
Identical datagram
Identical datagram
Network interface
Identical frame
Identical frame
Network interface
Network interface
Physical Net 1
Physical Net 2
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Internet datagram format
10
ICMP datagram format
11
ICMP Message types
Type Field ICMP Message Type
0 Echo
Reply 3
Destination Unreachable 4
Source Quench 5
Redirect (change a route)
8 Echo Request
11 Time Exceeded
for Datagram 12
Parametere Problem on a Datagram 13
Timestamp Request
14 Timestamp
Reply 15
Information Request (obsolete) 16
Information Reply
(obsolete) 17
Address Mask Request 18
Address Mask Reply
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Introduction to addressing

• Do I need an address?
• What types of addresses are there?
• Postal address
• Telephone number
• In Computer Networks
• Physical Addresses (Ethernet, FDDI, ...)
• Textual Addresses - Names
• Network level addresses (IP, X.25,...)

13
Addressing in the Internet

• Address specifies hosts interface
• 32 bit addresses
• Network part Host part
• Dotted decimal notation 192.164.2.4

14
Idea of routing

• Routers forward datagrams between connected
  networks
• They need to know via which interface to send a
  datagram
• Routing decisions are based on the information
  stored in the routing table

15
Routing table

• Tells where to send datagram for a particular
  network

Network Next-Hop
Port Metric
194.181.200.0 194.181.208.1 Eth0
1 193.2.1.0 194.181.208.320
Eth1 14 153.5.0.0
194.181.214.25 Fddi0 8 0.0.0.0
194.181.210.1 S0
5

• Next-Hop routers must be directly reachable

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Routing table (cont.)

• Default Route - a special entry in the routing
  table
• Pass all datagrams for unknown networks to this
  router
• Represented by the entry for network 0.0.0.0
• Routing uses network part of the address!

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Routing Algorithm

• Extract destination IP address from datagram
• Extract network address from the IP address
• If destination network equals my network
• Send directly to destination using physical
  network
• Else If destination address matches a
  host-specific route in the routing table
• Send to the router specified in the routing table

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Routing Algorithm (cont.)

• Else if destionation network matches a network in
  the routing table
• Send to the router specified in the routing entry
• Else If there is a default route in the routing
  table
• Send to the router specified in the default route
  entry
• Else
• Send a No route to host message to the source

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Populating the Routing Table

• Manually by network administrator Static Routes
• No dynamic changes to these routes will accur
• Dynamically by routing protocol
• Routing info is exchanged between routers
• The routing metric is used to find the best
  path

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Static Routes
A
B
Manually configured by network administrator
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Static Routes
A
B
Router cannot automatically reroute if path fails
22
Routing protocols

• Routers use a common protocol to exchange routing
  information
• Best path between networks or subnets is
  determined by Routing Metric
• Automatic adaption to topology changes

23
Routing protocols
64 kbps
64 kbps
2 Mbps
2 Mbps
24
Special address conventions

• Broadcast Addresses
• Directed broadcast host part all 1s -
  194.181.200.255
• Limited broadcast all 1s - 255.255.255.255
• 0 means This
• host part 0 - this host
• network part 0 - this network
• miss used as a broadcast address

25
Special address conventions (cont.)

• Loopback Address 127.0.0.1
• for testing and inter-process communication on
  the local machine
• should never appear on any network

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Summary of special address conventions
This host
Host on this net
Limited broadcast (local net)
Directed broadcast for net
Loopback
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Classess and address formats
0 1 2 3 4
8
16
24
31
netid
hostid
Class A
0
netid
hostid
Class B
0
1
netid
hostid
Class C
0
1
1
multicast address
Class D
0
1
1
1
reserved for future use
Class E
0
1
1
1
1
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Classes How to recognize them

• Class A first byte in range 1-126
• Class B first byte in range 128-191
• Class C first byte in range 192-223
• Class D first byte in range 224-239
• Class E first byte in range 240-255

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Classes Size and Number

• Class A 16.777.214 hosts, 128 networks
• Class B 65.534 hosts, 16.324 networks
• Class C 254 hosts, 2.097.152 networks

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Problems with Classes

• Class A usually to big
• Class C often to small
• Not enough Class Bs
• Inefficient utilisation of address space
• Solution extending the network part of the
  address Subnetting

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Subnetting
Class B Address Before Subnetting
Class B
0
1
Class B Address After Subnetting
Class B
0
1
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Subnet mask

• Subnet mask defines the network part
• binary 1 in network bits
• binary 0 in hosts bits
• Subnet mask must be contiguous!

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Subnetting (cont.)

• Not limited to byte border
• Subnets 0 and -1 used to be reserved
• Subnet 0 this subnet
• Subnet -1 broadcast
• Network administrator decides on the subnet size
• Network and subnet numbers used for routing
  decisions

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Subnetting and routing

• one subnet mask per particular network
• routing considerations
• all subnets of the same network must be
  contiguous
• or static routes must be used
• or routing protocol must carry also subnet masks

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Subnetting and routing

• all subnets of the same network must be
  contiguous!

C1
C1
C12
C11
B
C13
C14
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Subnet mask bits
128 64 32 16 8 4 2 1
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Binary Numbers
128 64 32 16 8 4 2 1
6
2
Represent 226 decimal in binary
1 1 1 0 0 0 1 0

2
6
128 64 32
2
226

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Subnetting a Class C
split
subnet mask
subnets
hosts/subnet
total hosts
utilis.
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Variable Length Subnet Masks (VLSM)

• Subnets are of different size
• A means for conserving address space
• How to do it
• how big is the biggest subnet?
• split the class into such pieces
• split (sub-subnet ) those peieces further

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VLSM (cont.)

• How to do VLSM

0

255
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VLSM and routing

• Prerequisites
• routing protocol must carry subnet masks
• or static routes must be used

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Classfull Addressing drawbacks

• Classfull Addressing Subnetting
• at least one route per class is advertised in
  routing updates
• Number of networks is doubling faster than once
  per year
• Memory is not growing that fast
• Only a few routers were able to keep the such
  number of routes
• Route flapping

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Classless addressing

• Introduced by CIDR - Classless InterDomain
  Routing
• Networks are grouped (aggregated) into blocks
• Blocks of networks are advertised
• New way of thinking
• there are no networks numbers, but just address
  space prefixes
• there are no subnet masks, just prefix lenghts

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Classless addresses notation

• 10.181.215.32 /27
• 10.181.215.32 with mask 255.255.255.224
• binary representation of mask
  11111111.11111111.11111111.11100000

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Classless address notation
Hosts . . . 8 16 32 64 128 256 . .
. 4096 8192 16384 32768 65535 . . .
Prefix . . . /29 /28 /27 /26 /25 /24 . .
. /20 /19 /18 /17 /16 . . .
Classful . . . 1 C . . . 16 Cs 32 Cs 64
Cs 128 Cs 1 B . . .
Subnet Mask . . . 255.255.255.248 255.255.255.240
255.255.255.224 255.255.255.192 255.255.255.128 2
55.255.255.0 . . . 255.255.240.0 255.255.224.0 255
.255.192.0 255.255.128.0 255.255.0.0 . . .
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Classless network aggregation - Supernetting

168
0
192
64
Class C 24-bit prefix
11000000
10101000
01000000
00000000
00000000
Common prefix 23 bits
11111111
11111111
11111110
00000000
0 00000000
168
/23
192
64
Classless 23-bit prefix
11000000
10101000
01000000
00000000
0 00000000
Prefix
Host part
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Classless network aggregation (cont.)

• Before aggregation
• 201.222.191.0/24
• 201.222.192.0/24
• 201.222.193.0/24
• After aggregation
• 201.222.191.0/24
• 201.222.192.0/23

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Classless addressing and routing

• Longest match routing
• Route distr. between two protocols, one is not
  supporting classless
• use a default route
• explode supernet info. into individual network
  numbers

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Classes of routing protocols

• The early Arpanet was completelly flat - single
  network model
• one routing protocol, all routers had all the
  routing info
• with the growth it become hard to maintaine and
  computationally intensive
• Solution split the Internet into a set of
  Autonomous Systems (AS)
• Each Autonomous System is a set of routers and
  networks under the same administration

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Classes of routing protocols (cont.)

• Special routers, called Exterior gateways used
  to connect ASes
• Two classes of routing protocols
• Interior routing protocols (IGP - Interior
  Gateway protocols)
• Exterior routing protocols (EGP - Exterior
  Gateway protocols)

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Interior Routing Protocols (IGPs)

• Used inside an Autonomous System
• Designed to handle more redundant links
• Links are cheaper in a local environment gt one
  can afford more redundant links
• Designed with a higher bandwidth in mind
• Cheaper bandwidth gt one can use more bandwidth
  for the exchange of routing information

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Interior Routing Protocols (cont.)

• They generally contaion less ingformation than
  EGPs
• IGPs in general (with exeptions) do not have to
  know about any other network outside the AS
• No policy support
• Inside AS, one generally does not want to aplly
  policy
• everyone can use every available link
• policies are generally only set on what links
  should be preffered

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Interior Routing Protocols (cont.)

• Fairly extensive metric support
• Redudancy gt one has to distinguish between
  redundant links
• metrics or costs help in the decision proccess
• Designed for fast convergence
• Because of the redudancy, IGPs are designed to
  make quick changes if the network topology changes

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Exterior Routing Protocols (EGPs)

• Used to exchange routing information between ASes
• Designed with lower bandwidth in mind
• long distance links are more expensive gt routing
  protocol should use less bandwidth for the
  exchange of routing information
• They generally contain a lot of information
• EGPs have to know about all external networks
• In the Internet that might be 40.000 networks

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Exterior Routing Protocols (cont.)

• They assume a less reliable network
• most of them are connection oriented for reliable
  delivery
• They are designed to provide policy control
• generally you set routing policy at the border
  of your routing domain
• They do not run in every single router
• Only at the border of your AS you have to run an
  EGP
• Internal routers can be less powerfull

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Summary

• We have covered
• Internet topology
• Routing
• static, dynamic
• classes of routing protocols
• Addressing
• classfull
• subnetting
• VLSM
• classless

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Where to get more information

• RFCs (RFC-1880 Internet Official Protocol
  Standards)
• Books
• D.C.Lynch, M.T.Rose Internet System Handbook
• D.E.Comer Internetworking with TCP/IP
• Mailing lists
• Usenet News