Computer Networks I IP addressing

Version Mar 14, 2011
Computer Networks I
application
transport
network
IP addressing
link
physical
David.Villa@uclm.es

Computer Networks I 2
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Introduction
Special addresses
Classful addressing
Private addresses
Subneting
VLSM
Superneting
Classless addressing
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CIDR
Outline

Computer Networks I 3
Introduction
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●
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The IP address is a 32 bit integer.
The address identifies a connection point (a NIC).
IP addresses are universally unique.
● The addressing space is 2 32 = 4.294.967.296
●
A single host may have several interfaces and it
must have an address per connected interface.

Computer Networks I 4
Introduction
●
The IP address is usually represented as a
sequence of 4 decimal numbers (0-255) separated
with dots.
161.67.136.169
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But it may be represented as a binary sequence:
10100001.01000011.10001000.10101001

Computer Networks I 5
Introduction
The IP address has two parts
●
●
NetID, identifies the network (designed by a
global authority) IANA (Internet Assigned
Number Authority)
HostID, identifies a host inside the network.
subnet
host
32 bits

Computer Networks I 6
Special addresses
RFC
3330
0.0.0.0
This host (any interface)
11111111.11111111.11111111.11111111 All hosts: 255.255.255.255
XX ... XX
00 ... 00
This network
XX ... XX
11 ... 11
All hosts in this network
00 ... 00 XX ... XX A host in this network
01111111.X.X.X Loopback: 127.0.0.1

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Classful Addressing
There are 5 classes, recognized by the MSB bits.
RFC
791
0
0 1 2 3 4 5 6 7 8 9
1
0 1 2 3 4 5 6 7 8 9
2
0 1 2 3 4 5 6 7 8 9
3
0 1
0 net id
host id
A class
10 net id
host id B class
110 net id
host id C class
1110 multicast groups
D class
1111 reserved for future use E class

Computer Networks I 8
Classful Addressing
Classes:
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A: 2 31 addresses (50%) 1.0.0.0 - 127.255.255.255
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B: 2 30 addresses (25%) 128.0.0.0 - 191.255.255.255
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C: 2 29 addresses (12,5%) 192.0.0.0 - 223.255.255.255
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D: 2 28 addresses (6,25%) 224.0.0.0 - 239.255.255.255
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E: 2 28 addresses (6,25%) 240.0.0.0 - 255.255.255.255
The IANA (Internet Assigned Numbers Authority) assignees address
blocks. IANA depends on ICANN (Internet Corporation for Assigned
Names and Numbers).
i
IPv4 address block assignments in:
http://www.iana.org/assignments/ipv4-address-space/ipv4-address-space.xml

Private addresses
RFC
1918
Some blocks are reserved for private addressing (privately
administered). The packets with a private destination
address must be not forwarded
outside the network.
10.0.0.0 -10.255.255.255/8
●
(16.777.216 hosts in 1 block)
172.16.0.0 - 172.31.255.255/12
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(1.048.576 hosts in 16 blocks)
192.168.0.0 - 192.168.255.255/16
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(65.536 hosts in 256 blocks)
http://xkcd.com/742/
Computer Networks I 9

Computer Networks I 10
Link-local addresses
RFC
3927
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It is a method to auto-assign an IP address without
DHCP or other external service.
●
The host selects an address in the block
169.254/16 using a pseudo-random number.
●
●
These address are valid only to communicate with
neighbors (same physical or logical link).
These are not routable address.

Computer Networks I 11
Classful addressing
Network address (net id)
The network address:
●
●
is the first address of its block.
identifies the whole network from the Internet point
of view.
From any host address it is possible to know its
network address, class and address range of the
block.

Computer Networks I 12
Classful addressing
Network mask
The mask is a 32-bit integer that determines the net-id.
network address = host address AND mask
i
It is a B class network
161 67 38 13
1010 0001
0100 0011 0010 0110 0000 1101
mask
1111 1111 1111 1111 0000 0000 0000 0000
network address
1010 0001 0100 0011 0000 0000
0000 0000

Computer Networks I 13
Classful addressing
Network mask
Default masks for the three classes
A
B
C
255 0 0 0
255 255 0 0
255 255 255 0
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The mask is not required when we use classful addressing only.
Other way to indicate mask is “CIDR notation”.
Example: The mask 255.255.0.0 may be indicated like:
● 161.67.27.38 /16

Computer Networks I 14
Subnetting
RFC
950
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Problem: The A and B class networks are underutilized.
Solution: Divide them in smaller sub-networks. Part of the host id
is used to identify the sub-net.
0
0 1 2 3 4 5 6 7 8 9
10 net id
1
0 1 2 3 4 5 6 7 8 9
2
0 1 2 3 4 5 6 7 8 9
host id
3
0 1
i
RFC
1878
n bits -> 2 n subnet
sub-net id
● The scheme shows a 4-bit sub-net id. Therefore, there are 16
subnets with 2 12 -2 hosts each.

Computer Networks I 15
Subnetting
●
The number of subnets must be a power of 2.
●
●
●
It may be applied to any not-used block.
That is a local decision, taken by the administrator. It is not perceived from
outside.
Sample: Apply subnetting to the next network to obtain 4 blocks:
141.14.0.1 141.14.0.2 141.14.192.2
141.14.255.253 141.14.255.254
Red: 141.14.0.0
141.14.201.4
Conventional B-class network
Internet

Subnetting
sample (cont)
Computer Networks I 16
141.14.0.1 141.14.0.2
141.14.63.254 141.14.64.1 141.14.64.2
141.14.127.254
Subnet: 141.14.0.0/18
Subnet: 141.14.64.0/18
X.X.0000 0000.0 X.X.0100 0000.0
141.14.44.12
141.14.88.9
141.14.198.24
141.14.128.1 141.14.128.2
141.14.191.254
141.14.192.1 141.14.192.2
141.14.255.254
Subnet: 141.14.128.0/18
Subnet: 141.14.192.0/18
X.X.1000 0000.0 X.X.1100 0000.0
B-class network divided
In 4 subnets
141.14.167.20
Internet
141.14.201.4

Subnetting
Subnet mask
Computer Networks I 17
●
With subnetting routing is not possible without a mask that
defines the subnet.
● In the previous sample
host id
0
1
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9
10 net id
2
0 1 2 3 4 5 6 7 8 9
3
0 1
sub-net id
●
Mask is
1111 1111 1111 1111 11 00 0000 0000 0000

Subnetting
Variable Length Subnet Mask (VLSM)
Computer Networks I 18
●
●
●
●
If the mask has a fixed size, all the subnets are the same size.
This is a big issue because in many situations we need very
small blocks. The serial links require only 2 IP address!!
VLSM allows to apply subnetting in a nested way
VLSM requires specific support from dynamic routing
protocols. RIPv1 y GRP does not support VSLM, while RIPv2,
OSPF and EIGRP do.

Variable Length Subnet Mask (VLSM)
Sample 1
Divide the C-class network 200.10.10.0 in 3 subnets
with 120 and 2x60 hosts.
●
Subnet 0:
200.10.10.0/24:
● Net address: 200.10.10.0 /25
200.10.10.0/24:
● Holds 62 hosts
● Holds 126 hosts
● Subnet 1:
● Net address: 200.10.10.128 /26
● Holds 62 hosts
● Subnet 2:
● Net address: 200.10.10.192 /26
● 200.10.10.0/25
● 200.10.10.128/25
● 200.10.10.128/26
● 200.10.10.192/26
Computer Networks I 19

Variable Length Subnet Mask (VLSM)
Sample 2
Computer Networks I 20
172.16.1.0/24 172.16.2.0/24
Divide the network 172.16.14.0/24 to get 8 subnet with several sizes.
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172.16.14.0/26
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172.16.14.64/26
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172.16.14.128/26
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172.16.14.192/26
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172.16.14.192/27
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172.16.14.224/27
– 172.16.14.224/30
– 172.16.14.228/30
– 172.16.14.232/30
– 172.16.14.236/30
– 172.16.14.240/30
– 172.16.14.244/30
– 172.16.14.248/30
– 172.16.14.252/30
172.16.14.0/26
172.16.14.64/26
172.16.14.128/26
172.16.14.192/27
172.16.14.224/30
172.16.14.228/30
172.16.14.232/30
172.16.14.236/30
172.16.14.0/24

Computer Networks I 21
Supernetting
RFC
1338
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Problem: C-class network are too small.
Solution: Aggregate small networks to get largest. Part of the net-id
is used to address hosts.
0
0 1 2 3 4 5 6 7 8 9
1
0 1 2 3 4 5 6 7 8 9
2
0 1 2 3 4 5 6 7 8 9
3
0 1
110 net id
host id
Requirements:
● The numbers of blocks must be power of 2.
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The block must have continuous addressing.
The third byte of the first address must be divisible by the number of
blocks.

Supernetting
Super-net mask
Computer Networks I 22
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It requires a net mask to make routing possible.
Super-net mask
1111 1111 1111 1111 1111 1 000
0000 0000
Supernetting
- 3 bits
C-class default mask
1111 1111 1111 1111 1111 1111 0000 0000
Subnetting
+ 3 bits
Sub-net mask
1111 1111 1111 1111 1111 1111 1110
0000

Computer Networks I 23
Classless addressing
●
Classfull address is little flexible.
● Classless allows to define blocks of any size (power of 2).
●
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●
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It is a generalization of subnetting. The same
requirements are applied.
The first address and the mask defines the block.
Supernetting has no sense in classless addressing
It implies routing problems, solved by CIDR.

Computer Networks I 24
Classless Inter-Domain Routing (CIDR)
RFCs
1518 a
1520
●
●
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CIDR is based in the definition of net-id prefix of
any size.
The first level assignment authority is IANA. There
are 5 RIR (Regional Internet Registry) that spread
addressing space among minor entities.
CIDR uses VLSM to define arbitrary size sub-nets.
With CIDR, the routers must change the way the
use their routing tables, all address requires
always a mask.

Computer Networks I 25
References
●
B.F. Transmisión de datos y redes de comunicaciones, cuarta edición
2007.
●
Chapter 19
●
A.S. Redes de computadores. Pearson Educación, Cuarta edición, 2003.
●
Pages 438 - 444
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Behrouz A. Forouzan. TCP/IP Protocol Suite. McGraw-Hill, 2003.
●
Chapter 5 and Section 6.6
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CISCO Systems. Inc. Guía del primer año. CCNA 3 y 4.Cisco Press,
2003.
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Chapter 2
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Cited RFCs.