CS 372 – introduction to computer networks*
Friday July 23, 2010
Announcements:
 Midterms are graded.
 Lab 4 is posted.
* Based in part on slides by Bechir Hamdaoui and Paul D. Paulson.
Acknowledgement: slides drawn heavily from Kurose & Ross
Chapter 4, slide: 1
Internet addresses
 A key aspect of a virtual network is a single,
uniform address format
 Can't use hardware addresses because
different technologies have different
address formats
 Can't use addresses that are local to a
network because multiple networks might use
the same addresses internally
 Address format must be independent of any
particular hardware address format
Chapter 4, slide: 2
IP address notation
 IP address is just a 32-bit number
 It’s the same internally, regardless of its external
representation
 In decimal form, the range is [0 … 4294967296]
 Some are reserved
 In hexadecimal form, the range is [00000000 … FFFFFFFF]
•
•
•
•
divides naturally into 4 2-hexdigit groups
e.g.: 80 FD 28 1C
each group represents one byte (octet)
Binary form is 10000000 11111101 00101000 00011100
 For convenience, use dotted decimal notation
 e.g.:
128.253.40.28 (= 80.FD.28.1C)
 range is [0.0.0.0 … 255.255.255.255]
Chapter 4, slide: 3
IP Addressing: introduction
 IP address: 32-bit
identifier for


223.1.1.1 = 11011111 00000001 00000001 00000001
host,
router interface
223
1
1
1
223.1.1.1
 interface: connection
between host/router and
physical link



multiple interfaces per
router
one interface per host
one IP address per
interface
223.1.2.1
223.1.1.2
223.1.1.4
223.1.1.3
223.1.3.1
223.1.2.9
223.1.3.27
223.1.2.2
223.1.3.2
Chapter 4, slide: 4
Subnets
 IP address:
 subnet part (higher bits)
 host part (lower bits)
subnet
part
host
part
223.1.1.1
223.1.2.1
223.1.1.2
223.1.1.4
223.1.1.3
11001000 00010111 00010000 00000000
223.1.3.27
223.1.2.2
subnet
200.23.16.0/23
223.1.3.1
 What’s a subnet ?
 device interfaces with same
subnet part of IP address
 can physically reach each
other without intervening
router
223.1.2.9
223.1.3.2
network consisting of 3 subnets
Chapter 4, slide: 5
Subnets
Recipe
 To determine the
subnets, detach each
interface from its
host or router,
creating islands of
isolated networks.
Each isolated network
is called a subnet.
223.1.1.0/24
223.1.2.0/24
223.1.3.0/24
Subnet mask: /24
Chapter 4, slide: 6
Subnets
223.1.1.2
How many?
223.1.1.1
223.1.1.4
223.1.1.3
223.1.9.2
223.1.7.0
223.1.9.1
223.1.7.1
223.1.8.1
223.1.8.0
223.1.2.6
223.1.2.1
223.1.3.27
223.1.2.2
223.1.3.1
223.1.3.2
Chapter 4, slide: 7
IP addresses: how to get one?
Q: How does host get IP address?
 hard-coded by system admin in a file
 DHCP: Dynamic Host Configuration Protocol:
 dynamically get IP address from as server when joining
the network
 IP address can be reused by other hosts if released
 Can renew IP addresses if stayed connected
Chapter 4, slide: 8
DHCP client-server scenario
A
B
223.1.2.1
DHCP
server
223.1.1.1
223.1.1.2
223.1.1.4
223.1.2.9
223.1.2.2
223.1.1.3
223.1.3.1
223.1.3.27
223.1.3.2
E
arriving DHCP
client needs
address in this
network
Chapter 4, slide: 9
IP addresses: how to get one?
Q: How does network get subnet part of IP
addr?
A: gets allocated portion of its provider ISP’s
address space
ISP's block
11001000 00010111 00010000 00000000
200.23.16.0/20
Organization 0
Organization 1
Organization 2
...
11001000 00010111 00010000 00000000
11001000 00010111 00010010 00000000
11001000 00010111 00010100 00000000
…..
….
200.23.16.0/23
200.23.18.0/23
200.23.20.0/23
….
Organization 7
11001000 00010111 00011110 00000000
200.23.30.0/23
Chapter 4, slide: 10
IP addressing: the last word...
Q: How does an ISP get block of addresses?
A: ICANN: Internet Corporation for Assigned
Names and Numbers
 allocates addresses
 manages DNS
 assigns domain names, resolves disputes
Chapter 4, slide: 11
IP address format
 Each IP address is divided into a prefix and a
suffix
 Prefix identifies the network and the type of
network to which a host computer is attached
 Suffix identifies a host computer within that
network
 Usually includes indicator for number of bits
used for prefix
 Address format enables efficient routing
Chapter 4, slide: 12
IP address hierarchy
 Every network in a TCP/IP internet is
assigned a unique network number
 Each host on a specific network is assigned a
host number or host address that is unique
within that network
 Host's IP address is the combination of the
network number (prefix) and host address
(suffix)
Chapter 4, slide: 13
IP address assignment
 Network numbers (prefixes) are unique
 Host addresses (suffixes) may be duplicated
on different networks
 The combination of network number prefix
and host address suffix is unique in the entire
internet
Chapter 4, slide: 14
IP address assignment
 Assignment of network numbers must be
coordinated globally
 Assignment of host addresses can be
managed locally
Chapter 4, slide: 15
IP address design
 IP-v4 designers chose 32-bit addresses
 Allocate some bits for prefix, some for
suffix
Large prefix, small suffix - many networks, few
hosts per network
 Small prefix, large suffix - few networks, many
hosts per network

 Because of the wide variety of
technologies, need to allow for both large
and small networks
Chapter 4, slide: 16
IP addressing (two types):
Classful addressing: A, B, C



A: /8 (only 28 networks, but 224 hosts per network)
B: /16 (216 networks, and 216 hosts per network)
C: /24 (224 networks, but only 28 hosts per network)
CIDR: Classless InterDomain Routing


network portion of address of arbitrary length
address format: a.b.c.d/x, where x is # bits in network
portion of address
network
part
host
part
11001000 00010111 000100 00 00000000
200.23.16.0/22
Chapter 4, slide: 17
IP address classes:
Classful addressing
 Multiple address formats that allow both
large and small prefixes
 Each format is called an address class
 The class of an address is identified by
first four bits
 The number of bits allocated for the
prefix is determined by the class
Chapter 4, slide: 18
IP address classes:
Classful addressing
Chapter 4, slide: 19
Chapter 4, slide: 20
IP address classes:
Classful addressing
 Class A, B and C are primary classes
 Used for ordinary host addressing
 Class D is used for multicast, a limited form of
broadcast




Internet hosts join a multicast group
Packets are delivered to all members of group
Routers manage delivery of single packet from source to all
members of multicast group
Used for multicast backbone
 Class E is reserved
Chapter 4, slide: 21
Determining IP class
 Dotted decimal makes separating network
address from host address easier
 Look at first dotted decimal number, and use
this table:
Chapter 4, slide: 22
How many networks?
 Classful scheme does not yield equal number of
networks in each class
 class A:



First bit must be 0
7 remaining bits identify Class A net
27 (= 128) possible class A nets
• Minus a few that are reserved
 class B:
 First 2 bits must be 10
 14 remaining bits identify Class B net
 214 (= 16384) possible class B nets
• Minus a few that are reserved
 class C:
 First 3 bits must be 110
 21 remaining bits identify Class C net
 221 (= 2097152) possible class C nets
• Minus a few that are reserved
Chapter 4, slide: 23
Networks and hosts in each class
8
16
24
Chapter 4, slide: 24
What if the form doesn't fit?
 Large organizations may not be able to get as
many addresses in the Internet as they need
 Example - UPS needs addresses for millions of
computers
 Example – School needs 6000 hosts

Too big for class C, too many wasted addresses for
class B
Chapter 4, slide: 25
Possible solutions
 Classless addressing

allow division between prefix and suffix at any bit
boundary
 Sharing an IP address

Use one IP address for multiple hosts
Chapter 4, slide: 26
Classless Addressing Example
128.193.47.25
dotted decimal
80 C1 2F 19
hexadecimal
1000 0000 1100 0001 0010 1111 0001 1001 binary
This can be “re-aligned” to use variable-size prefixes and
suffixes.
Example: Suppose we want a 22-bit prefix and a 10-bit
suffix
Chapter 4, slide: 27
Example:
 1000 0000 1100 0001 0010 1111 0001 1001

Logical AND with a "mask" using 22 bits for
prefix (netmask), 10 bits for suffix:
1111 1111 1111 1111 1111 1100 0000 0000
 … gives a prefix:
1000 0000 1100 0001 0010 1100 0000 0000
 The complement mask (hostmask)
0000 0000 0000 0000 0000 0011 1111 1111
 … gives a suffix:
0000 0000 0000 0000 0000 0011 0001 1001
Chapter 4, slide: 28
Example:
1000 0000 1100 0001 0010 1100 0000 0000
0000 0000 0000 0000 0000 0011 0001 1001
i.e., the “network number” is 80C12C00h, and the “host
number” within the network is 319h
 With 10 bits for suffix, 1024 host addresses are
available within the subnet (but 2 of these are
reserved)
 Address still looks almost the same in dotted
decimal
128.193.47.25 / 22
 Additional information is provided so addressing can be
handled by routers

Chapter 4, slide: 29
CIDR
 CIDR (Classless Inter-Domain Routing) address
includes specification for number of bits to use
for the netmask





Example: host address 128.193.47.25/22
What is the netmask?
• 255.255.252.0
What is the network address?
• 128.193.44.0
What is the hostmask?
• 0.0.3.255
What is the host number?
• 0.0.3.25 = 319h = 793 (decimal)
Chapter 4, slide: 30
CIDR
 What is the netmask for /20 ?

255.255.240.0
 What is the netmask for /24 ?
 255.255.255.0
 What is the netmask for /27 ?

255.255.255.224
 How many hosts can be supported in /28 ?
 24 - 2 = 14
Chapter 4, slide: 31
Example
 Three subnets
 All interfaces in all these
Subnet 1
Subnet 2
subnets are required to have
prefix: 223.1.13/24
 Subnet 1 is required to
support 125 interfaces
 Subnet 2 & 3 are each
required to support 60
interfaces
 Question:
Provide 3 network addresses
in the form: a.b.c.d/x
Subnet 3
Chapter 4, slide: 32
IP address assignment
 Select an address class for each network depending
on expected number of hosts
 Assign network numbers from appropriate classes
 Assign host suffixes to form internet addresses for
all hosts
 The lowest host number (0) is not used because that
complete address is the network address

e.g., 192.168.5.0
 The highest host number (depends on network type)
is not used because that complete address is the
“broadcast address” for the network

e.g., 192.168.5.255
Chapter 4, slide: 33
OSU IP addresses
 Oregon State has a single Class B network:
128.193.0.0
 All hosts at OSU have 128.193 prefix
 E.G.:
ns1.oregonstate.edu 128.193.0.10
 ns2.oregonstate.edu 128.193.4.20

 Suffix bytes are used to determine local
network and host through subnetting
 Individual host addresses assigned by system
administrators

may be static or dynamic assignment
Chapter 4, slide: 34
IP address allocation
 Addresses in the Internet are not used
efficiently

Less than 10% of possible addresses are actually
assigned
 Concerns about address space being
exhausted
 OSU is like most organizations, using 5,0006,000 out of possible 216 (= 65,536) available
addresses
Chapter 4, slide: 35
Routing table information
 In the routing table
Destination stored as network address
 Next hop stored as IP address of router

 Address mask defines how many bits of
address are in prefix
Prefix defines how much of address used to
identify network
 e.g., class B mask is 255.255.0.0

• In binary: 11111111111111110000000000000000
Chapter 4, slide: 36
Example:
Routing table
for
Chapter 4, slide: 37
Consider 192.4.10.26:
Routing table
for
doesn’t care what
does with the packets
Chapter 4, slide: 38
Address masks
 To identify destination network, apply address
mask to destination address and compare to
network address in routing table
 Use Boolean and
if ((Mask[i] & D) == Dest[i])
forward to NextHop[i]
Chapter 4, slide: 39