Internet Protocol vesion-4 (IPV4)

The frame transmitted is of following type where the type field indicates what the next header will be and where it is to be processed. If the type field is 0x800 then it is IP and the next layer network/internet layer should process it


Figure-1: Ethernet Frame
  • Prior to discarding the frame header and trailer, it is necessary for the next set of instructions to be processed to be determined from the frame header. As highlighted, this is identified by determining the field value in the type field, which in this instance represents a frame that is destined for the IP protocol following completion of the frame process.
  • The key function of the frame is to determine whether the intended physical destination has been reached, that the integrity of the frame has remained intact. The focus of this section will identify how data is processed following the discarding of the frame headers and propagation of the remaining data to the Internet Protocol.

IP Packet Header

The IP header is carried as part of the data and represents an overhead of at least 20 bytes that references how traffic can be forwarded between networks. The IP header is used to support two key operations, routing and fragmentation. The source and destination IP addressing are logical 32 bit addresses assigned to hosts. Routing is the mechanism that allows traffic from a given network to be forwarded to other networks, since the data link layer represents a single network for which network boundaries exist.

Figure-2: IPv4 format


  • Protocol− Tells the Network layer at the destination host, to which Protocol this packet belongs to, i.e. the next level Protocol. For example protocol number of ICMP is 1, TCP is 6 and UDP is 17.
  • Options− This is optional field, which is used if the value of IHL is greater than 5. These options may contain values for options such as Security, Record Route, Time Stamp, etc
  • Time to Live− To avoid looping in the network, every packet is sent with some TTL value set, which tells the network how many routers (hops) this packet can cross. At each hop, its value is decremented by one and when the value reaches zero, the packet is discarded.
  • Fragmentation – It refers to the breaking down of data into manageable blocks that can be transmitted over the network.
  • version – The version field here is IPv4.
  • DS field – The DS field is the type of service field however now operates as a field for supporting differentiated services, primarily used as a mechanism for applying quality of service (QoS) for network traffic optimization, and is considered to be outside of the scope of this training.

IP Addressing

a. Network Address

Network Address
Host Address

b. Broadcast Address

Network Address
Host Address
  • Each IPv4 address represents a 32 bit value that is often displayed in a dotted decimal. binary or Hex format as shown in figure.
  • Each network range contains two important addresses that are excluded from the assignable network range to hosts or other devices these are the network address is identified by .0 and broadcast address is identified by .255 (.FF) in least byte.
  • So the effective range of host address in the network are 254 from address .1 to .254 as the least significant byte.

IP Address Classes

Network Address
Host Address
Class A


Host 24-bits
Class B 10 Network (16bit)


Host (16bit)


Class C

110      Network (24bit)

Host (8bit)


Class D

1110  Multicast

Class E

1111 Experimental

  • Classes A, B and C are assignable address ranges, each of which supports a varied number of networks, and a number of hosts that are assignable to a given network.
  • Class A for instance consist of 126 potential networks, each of which can support 224, or 16’777’216 potential host addresses, bearing in mind that network and broadcast addresses of a class range are not assignable to hosts. In truth, a single Ethernet network could never support such a large number of hosts since Ethernet does not scale well, due in part to broadcasts that generate excessive network traffic within a single local area network.
  • Class C address ranges allow for a much more balanced network that scales well to Ethernet networks, supplying just over 2 million potential networks, with each network capable of supporting around 256 addresses, of which 254 are assignable to hosts.
  • Class D is a range reserved for multicast, to allow hosts to listen for a specific address within this range.
  • Class E is for experimental purpose.

IP Address Types

Private Address Ranges
Class A  and assignable address are to
Class B
Class C
  • Private address ranges exist within the class A, B and C address ranges to prolong the rapid decline in the number of available IP addresses.
  • The number of actual end systems and devices that require IP addressing in the world today exceeds the 4’294’967’296 addresses of the 32 bit IPv4 address range, and therefore a solution to this escalating problem was to allocate private address ranges that could be assigned to private networks, to allow for conservation of public network addresses that facilitate communication over public network infrastructures such as the Internet.

Private networks have become common throughout the enterprise network but hosts are unable to interact with the public network, meaning that address ranges can be reused in many disparate enterprise networks. Traffic bound for public networks however must undergo a translation of addresses before data can reach the intended destination.

Special Addresses
Diagnostic ~
Any Network
Network Broadcast for the IPv4 ( network, however the scope of any broadcast in IP is restricted to the boundaries of the local area network from which the broadcast is generated


  • Other special addresses include :
  • a diagnostic range denoted by the network address
  • Any Network is represented by
  • Broadcast address as


IP Communication

Figure : IP Communication
  • For communication
  • A host should have knowledge of the destination network.
  • A host is naturally aware of the network to which it belongs. In the case where the intended destination network varies from the originating network, the host is expected to have knowledge of the intended network and the interface via which a packet/frame should be forwarded before the packet can be processed by the lower layers.
  • As such hosts will not forward data intended for a given destination until the host learns of the network and thus with it the interface via which the destination can be reached.

Subnet Mask

  • Subnet mask is used for dividing the network into small segments.
  • It is the mask value that governs the identification of a unique network segment.
  • A mask value that is used to distinguish the number of bits that represent the network segment, for which the remaining bits are understood as representing the number of hosts supported within a given network segment.
  • A network administrator can divide a network address into sub-networks so that broadcast packets are transmitted within the boundaries of a single subnet.
  • The subnet mask consists of a string of continuous and unbroken 1 values followed by a similar unbroken string of 0 values. The 1 values correspond to the network ID field whereas the 0 values correspond to the host ID field.
Network Host
192.168.1 .0
11000000.10101000.000000001 00000000
255.255.255 .0
11111111.11111111.11111111 .00000000

Default Subnet mask

For each class of network address, a corresponding subnet mask is applied to specify the default size of the network segment. Table below shows the default subnet mask for each class of networks.

Subnet mask
Default subnet mask bits (least significant)
first octet of the IP address, with the remaining three octets remaining available for host ID assignment
first two octet of the IP address, with the remaining two octets remaining available for host ID assignment
The class C network defaults to a 24 bit mask that provides a large number of potential networks but limits greatly the number of hosts that can be assigned within the default network

Case Scenario

From the table below, determine the network for a given IP address, the number of actual and valid addresses in the network.

IP Address
Subnet mask

Sulution: To find the actual range of host address, we can AND the IP address and the subnet mask. This is shown below.

IP Address
Subnet mask
ANDING IP and subnet mask
10101100. 00010000. 00000000. 00000000
Network Address
No. of actual address
2^16 =64535
No. of valid host address
2^16 – 2 = 64533


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