1. Network Layer: IP Addressing
CE 306, IEU/Computer Eng.
S. Kondakci

2. Topics Covered Addresses for the Virtual Internet
The IP Addressing Scheme
The IP Address Hierarchy
Original Classes of IP Addresses
Dotted Decimal Notation
Address Masks
Division of the Address Space (Subnetting)
Subnet and Classless Addressing: CIDR Notation
Special IP Addresses
Routers and the IP Addressing Principle
Multi-Homed Hosts

3. Addresses for the Virtual Internet
All host computers must use a uniform addressing scheme
Each address of a node must be unique
MAC addresses do not suffice because
the Internet can include multiple network technologies
and each technology defines its own MAC addresses
The advantage of IP addressing lies in uniformity:
an arbitrary pair of application programs can communicate without knowing the type of network hardware or MAC addresses being used

4. The IP Addressing Scheme
When sending a packet across the Internet, sender’s protocol software must specify
its own 32-bit IP address (the source address)
and the address of the destination machine

5. Example Communication
Browser/client
DNS server
Another network
/13
Your network
/24
web page
Server/destination
Google’s network
/19

6. IP Address Structure & Hierarchy
Each host is assigned a unique 32-bit number known as the host's IP address or Internet address.
IP address is divided into two parts:
A prefix (Network part)
identifies the physical network to which the host is attached
Each network in the Internet is assigned a unique network number
A suffix (Host part)
identifies a specific computer (host/node) on the network
Each computer on a given network is assigned a unique suffix
IP address scheme guarantees two properties:
Each computer is assigned a unique address
Network number assignments must be coordinated globally
Suffixes are assigned locally without global coordination
bit bit 31
Prefix(Net-ID)
Suffix(Host-ID)

7. Original Classes of IP Addresses
The original classful IP addressing divides the IP address space into three (3) primary classes, A, B, and C
each class has a different size of prefix and suffix

8. Dotted Decimal Notation
express each 8-bit section of a 32-bit number as a decimal value
uses periods (dots) to separate the sections
The scheme is known as dotted decimal notation
Each octet (byte) is an unsigned binary integer
dotted decimal addresses range is through

9. Division of the Address Space
The classful scheme divides the address space into unequal sizes

10. IP Address Ranges

11. Subnetworking
As the number of distinct local networks grows, managing them can become a serious problem
Solution: Split a network into several parts (subnetworks) for internal use but still act like a single network to the outside world.
E.g., Split the 16-bit host address area into 6-bit subnet number plus 10-bit host number.
Two mechanisms are used to split the network:
Subnet addressing
Classless addressing
The two mechanisms are closely related
they can be considered to be part of a single abstraction:
instead of having three distinct address classes, allow the division between prefix/suffix on an arbitrary bit boundary

12. Subnets (cont’d)
Outside the network, subnetting is not visible so allocating a new subnet does not require contacting NIC or changing any external databases
Every router has tables for
Network IP addresses (network, 0)
Local host IP addresses (this-network, host)
When a packet arrives, the router checks its tables
If it is destined for a distant network, the packet is forwarded to the next router
If destined for a local host, it is sent directly to the destination
Subnet masks are AND’ed by the IP address to determine the subnet address 12

13. Subnetworking by Netmasks
How can an IP address be divided at an arbitrary boundary?
The classless and subnet addressing schemes require hosts and routers to store an additional piece of information:
a value that specifies the exact boundary between the network prefix and the host suffix
To mark the boundary, IP uses a 32-bit value
known as an address mask, also called a subnet mask or netmask
Why store the boundary size as a bit mask?
A mask makes processing efficient
Hosts and routers need to compare the network prefix portion of the address to a value in their forwarding tables
The bit-mask representation makes the comparison fast and efficient

14. Address Masks Suppose a router is given
a destination address, D
a network prefix represented as a 32-bit value, N
a 32-bit address mask, M
Assume the top bits of N contain a network prefix, and the remaining bits have been set to zero
To test whether the destination lies on the specified network, the router tests the condition:
N == (D & M)
The router
uses the mask with a “logical and (&)” operation to set the host bits of address D to zero (0)
and then compares the result with the network prefix N

15. Address Masks in Action
As an example:
Consider the following 32-bit network prefix: =
Consider a 32-bit mask: =
Consider a 32-bit destination address =
A logical AND between the destination address and the address mask extracts the high-order 16-bits

16. A campus network consisting of LANs for various departments.
Subnets: Example
A campus network consisting of LANs for various departments. 16

17. Using (Sub)net Masks Given a class C network of 204.15.5.0.
Create 8 subnets
Show subnet addressess
Show host address range on each subnet
|sub| host
Number of hosts on each subnet = = 2^5-2=30
Subnet Netmask Host address range
to 30
to 62
to 94
to 126
to 158
to 190
to 222
to 254 17 17

18. Overall Structure of the 8 Subnets

19. Examples on Ubuntu

20. Class B subnetting You have 172.16.0.0 As the network ID
Take 5 bits from the host ID and use them as subnet bits,
|subnet| host
Number of subnets = 2^5 = 32
Number of hosts on each subnet = 2^11-2= = 2046

21. Class A subnetting You have 10.0.0.0 As the network ID
Take 7 bits from the host ID and use them as subnet bits,
| subnet | host
Number of subnets = 2^7 = 128
Number of hosts on each subnet = 2^17-2= =

22. CIDR Notation Classless Inter-Domain Routing (CIDR)
The name is unfortunate because CIDR only specifies addressing and forwarding
Designers wanted to make it easy for a human to specify a mask
Consider the mask needed for the example in Figure 21.4b
It has 26 bits of 1s followed by 6 bits of 0s
In dotted decimal, the mask is:
The general form of CIDR notation is: ddd.ddd.ddd.ddd/m
ddd is the decimal value for an octet of the address
m is the number of one bits in the mask
Thus, one might write the following: /26
which specifies a mask of 26 bits
Figure 21.5 lists address masks in CIDR notation
along with the dotted decimal equivalent of each

23. A CIDR Example The binary value assigned to customer1 is:
Assume an ISP has the following block /16
Suppose the ISP has 2 customers
one customer needs 12 IP addresses and the other needs 9
The ISP can assign
customer1 CIDR: /28
customer2 CIDR: /28
both customers have the same mask size (28 bits), the prefixes differ
The binary value assigned to customer1 is:
The binary value assigned to customer2 is:
There is no ambiguity
Each customer has a unique prefix
More important, the ISP retains most of the original address block
it can then allocate to other customers

24. CIDR Host Addresses

25. A list of address masks in CIDR notation and in dotted decimal

26. Subnet and Classless Addressing
Assume an Internet service prvider (ISP) owns a class C prefix
Classful addressing assigns the entire prefix to one organization
With classless addressing
the ISP can divide the prefix into several longer prefixes
and assign each to a subscriber
Figure illustrates how classless addressing allows an ISP to divide a class C prefix into four (4) longer prefixes
each one can accommodate a network of up to 62 hosts
the host portion of each prefix is shown in gray
The original class C address has 8 bits of suffix
and each of the classless addresses has 6 bits of suffix
Assuming that the original class C prefix was unique
each of the classless prefixes will also be unique
Thus, instead of wasting addresses
ISP can assign each of the four (4) classless prefixes to a subscriber

27. Subnet and Classless Addressing

28. Special IP Addresses
IP defines a set of special address forms that are reserved
Special addresses are reserved and never assigned to hosts, for example,
Loopback Address : Loopback address ( ) is used to test network applications
It is used for preliminary debugging after a network application has been created

29. Special IP Addresses Loopback Address 127.0.0.1
A programmer can test the program logic quickly
without needing two computers and without sending packets across a network
IP reserves the network prefix 127/8 for use with loopback
The host address used with 127 is irrelevant
all host addresses are treated the same
programmers often use host number 1
so it makes the most popular loopback address
During loopback testing no packets ever leave a computer
the IP software forwards packets from one application to another
The loopback address never appears in a packet traveling across a network

30. Summary of Special IP Addresses
The following table summarizes the special IP addresses

31. Routers and the IP Addressing Principle
Each router is assigned two or more IP addresses
one address for each network to which the router attaches
To understand why, recall two facts:
A router has connections to multiple physical networks
Each IP address contains a prefix that specifies a physical network
A single IP address does not suffice for a router
because each router connects to multiple networks
and each network has a unique prefix
The IP scheme can be explained by a principle:
An IP address does not identify a specific computer
each address identifies a connection between a computer and a network
A computer with multiple network connections (e.g., a router) must be assigned one IP address for each connection

32. Multi-homed Hosts: Routers and the IP Addressing Principle

33. The Internet Control Message Protocol (ICMP)
Provides control messaging among the routers to monitor the operation of the Internet
Messages are encapsulated in IP packets
DESTINATION UNREACHABLE
TIME EXCEEDED
PARAMETER PROBLEM
SOURCE QUENCH
REDIRECT
ECHO REQUEST
ECHO REPLY
TIMESTAMP REQUEST
TIMESTAMP REPLY
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34. Internet Control Message Protocol
The principal ICMP message types.
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