Network Layer – IPv4
Dr. Sanjay P. Ahuja, Ph.D.
Fidelity National Financial Distinguished Professor of CIS
School of Computing, UNF
IPv4

Internet Protocol (IP) is the glue that holds the Internet together.

Communication in the Internet:
 Transport Layer takes a data stream and breaks them up into
packets (datagrams).
 An IP datagram can be up to 64 KB but in practice they are about
1500 bytes.
 Each IP datagram is routed through the Internet, possibly being
fragmented into smaller units as it goes.
 When all the fragments get to the destination machine they are
reassembled by the network layer into the original datagram,
which is handed to the transport layer.
IPv4

The IP datagram header has a 20 byte fixed part and a variable length
optional part.
IPv4

Version (4-bits): indicates version of the protocol the datagram belongs to.

IHL (4-bits): This field provides the length of the IP header. The length of the
header is represented in 32 bit words. Minimum value = 0101 (i.e. 5) which
corresponds to 5 * 4 = 20 bytes. Maximum value = 1111 (i.e. 15) which
corresponds to 15 * 4 = 60 bytes. So the options part of the header can be at
most 40 bytes.

Differentiated Services (8-bits): Corresponds to type of service. The first 3
bits of this field are priority bits and are ignored as of today. The next 3 bits
represent type of service and the last 2 bits are unused. The 3 bits that
represent type of service are: minimize delay, maximize throughput, and
maximize reliability.

Total Length (16-bits): This represents the total IP datagram length in bytes
(header + data). Maximum size = 64 K or 65535 bytes.

Identification (16 bits): Enables the destination host to determine which
datagram a newly arrived fragment belongs to. All fragments of a datagram
contain the same Identification value.
IPv4
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DF bit (1-bit): Don’t fragment (if destination is incapable of putting a
datagram fragments back together).
MF (1-bit): More fragments. All fragments except the last one have this bit
set to 1.
Fragment Offset (13-bits): Indicates where in the current datagram this
fragment belongs (213 = 8192 fragments per datagram and 8192 * 8 = 65536
bytes. Each fragment is a multiple of 8 bytes)
TTL (8-bits): Used to limit packet lifetime. Maximum lifetime = 255 seconds.
In practice, it just counts hops. Default = 64 hops, which is decremented each
time the packet is forwarded.
Protocol (8-bits wide): Tells IP which transport protocol to give the
datagram to (i.e. TCP or UDP).
Header Checksum (16-bits): Verifies the header.
Source and Destination Addresses (32-bits each): Indicate IP address
(network number and host number) of host.
Options (maximum 40-bytes): Presences of options indicated by IHL field.
Options include record route, timestamp, and strict source routing.
IP Fragmentation – An Example
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MTU: largest IP datagram that can be carried in a frame is called the
Max Transmission Unit or MTU
MTU (Ethernet): 1500 bytes
MTU (FDDI): 4500 bytes
MTU (Point-to-point link): 532 bytes (512 bytes data + 20 bytes IP
header.
Host H1
Router R1
Ethernet
ETH
IP
1400
Router R2
FDDI
FDDI
IP
Router R3
Pt.-to-pt,
1400
Host H2
Ethernet
P2P
IP
512
ETH
IP
512
P2P
IP
512
ETH
IP
512
P2P
IP
376
ETH
IP
376
IP Fragmentation – Example contd.

Un-fragmented Packet:
Ident. = x
MF = 0
Offset = 0
MF = 1
Offset = 0
MF = 1
Offset =
512
MF = 0
Offset =
1024
1400 data bytes
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Fragmented Packets:
Ident. = x
512 data bytes
Ident. = x
512 data bytes
Ident. = x
376 data bytes
IPv4 Address Classes
IPv4 Address Classes

IP addresses are hierarchical, i.e. made up of 2 parts: a network part and a
host part.

Class A: has 7-bits for network part and 24 bits for the host part. There can
only be 27 = 128 class A networks and up to 224 – 2 = (16,777,214 or 16 million)
hosts.

Class B: 214 = 16,384 class B networks with up to 216 - 2 = 65534 hosts each.

Class C: 216 = 2 million class C networks with up to 28 - 2 = 254 hosts each.
Subnetting

Original intent was that one IP address uniquely identify one physical
network.

Subnetting is a way to reduce the total number of network numbers that are
assigned.

A network is split into several parts for internal use but still acts like a single
network to the outside world. Each part is a subnet.

A company starts with a class B address 128.64.0.0.
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The 16-bit host number is split into a 8-bit subnet number and a 8-bit host
number.
10
11
255.255.255.0
Subnet Mask
Network
11111111111111
Subnet
Host
1 1 1 1 1 1 1 1 00000000
Subnetting
10
11
255.255.255.0
Subnet Mask
Network
11111111111111
Subnet
Host
1 1 1 1 1 1 1 1 00000000
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This split allows 256 – 2 = 254 LANs, each with up to 254 hosts.

All hosts on the same LAN will have the same subnet number. Hosts on the
different LANs will share the same network number.

We can think of an address as having three parts: network part, subnet part,
and a host part. Subnetting introduces another level of hierarchy into the IP
address.
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128.64.1.254 AND 255.255.255.0 = 128.64.1.0 (LAN/Subnet #)