1. Network Layer – IPv4 Dr. Sanjay P. Ahuja, Ph.D.
Fidelity National Financial Distinguished Professor of CIS
School of Computing, UNF 1

2. 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.

3. IPv4
The IP datagram header has a 20 byte fixed part and a variable length optional part.

4. 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 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.

5. IPv4
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 = 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.

6. IP Fragmentation – An Example
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 R Router R Router R Host H2
Ethernet FDDI Pt.-to-pt, Ethernet
ETH IP
1400
FDDI IP
1400
P2P IP
512
ETH IP
512
P2P IP
512
ETH IP
512
P2P IP
376
ETH IP
376

7. IP Fragmentation – Example contd.
Un-fragmented Packet:
Fragmented Packets:
Ident. = x
MF = 0
Offset = 0
1400 data bytes
Ident. = x
MF = 1
Offset = 0
512 data bytes
Ident. = x
MF = 1
Offset = 512
512 data bytes
Ident. = x
MF = 0
Offset = 1024
376 data bytes

8. IPv4 Address Classes

9. 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 = 126 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 = hosts each.
Class C: 216 = 2 million class C networks with up to = 254 hosts each.
For Class A only: Network address 0 cannot be used and network address 127 is reserved for loop-back testing.
For all address classes, need to subtract 2 from the host number as all 0’s means this host and 1’s is a broadcast to all hosts within that network.
The class A network address block contains 2 31 power (2,147,483,648) individual addresses. The IPv4 address space contains a maximum of 2 32 power (4,294,967,296) addresses, which mean that a class A network address space is 50% of the total IPv4 unicast, address space.
This class defines 16,384 (2 14 ) /16 networks, and supports a maximum of 65,534 (2 16 -2) hosts per network. Class B /16 block address is (1,073,741,824) = 2 30; therefore it represent 25% of the total IPV4.
This class defines a maximum of 2,097,152 (2 21 ) /24 networks. And each network supports up to 254 (2 8 -2) hosts. The entire class C network represents 2 29 (536,870,912) addresses; therefore it is only 12.5 % of the total IPv4.

10. 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
The 16-bit host number is split into a 8-bit subnet number and a 8-bit host number.
Subnet Mask 10
Network
Subnet
Host

11. Subnetting
Subnet Mask
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.
AND = (LAN/Subnet #) 10
Network
Subnet
Host