Friday, February 28, 2014

IPv4 Network Classes

IPv4 addresses are divided into three groups:


Unicast Addresses

It includes Class A, B and C. Firs octet most significant bit identifies the class of IP address.
  • Class A IP addresses first octet start with 1-126.The first octet has a format 0xxxxxxx,where 0 identifies that it is Class A. 
  • Class B IP addresses first octet start with 128-191.The first octet has format 10xxxxxx, where 10 identifies that it is class B.
  • Class C IP addresses first octet start with 192-223.The first octet has format 110xxxxx, where 110 identifies that it is class C.

Multicast Addresses

It includes Class D.
  • Class D IP addresses first octet start with 224-239.

Experimental Addresses

It includes Class E. These addresses are only used for experimental purposes and its first Octet start with

Table lists the different facts about Class A, B and C networks.

  • Network Number/ID is the lowest number in the network
  • Total networks are the available number of networks in one class, it can be calculated by 2^network_bits. It must be noted that network bits are always one, two and three bit less; it is because the Class A reserve 1-bit, Class B reserve 2-bits and Class C reserve 3-bits for the identification of class.
  • The mask communicates the network and host parts of the IP address to the computer, therefore; every Class has its associated default subnet mask.
  • Broadcast Address of a Class is the numerically highest number in network. Routers forward a copy of packet with broadcast address as destination to all hosts in Classful network.
  • In between Network ID and Network Broadcast Address is the range of Usable IP Addresses. It can be calculated by 2^h-2. Two addresses (namely; network ID and broadcast address) are subtracted from the total available addresses as there addresses can't be used for hosts/nodes.

To find then network ID, Network Broadcast and Usable Address for an IP Address follow the following steps: 

  1. Determine class of the IP address by examining first octet.
  2. Divide the IP address into network and host part.
  3. To find network ID, change host part octets to 0.
  4. To find broadcast address, change network ID's host octets to 255.
  5. To find first usable address, add 1 to the fourth octet of the network ID.
  6. To find last useable address, subtract 1 from the fourth octet of the network broadcast address.
Example (
  1. Class C
  2. Network Part (223.223.0), Host Part (1)
  3. Network ID =
  4. Broadcast Address =
  5. First Usable Address =
  6. Last Usable Address =

Reserved Network IDs

  • Class A network was reserved for broadcasting requirements.
  • Class A network is reserved for loopback addresses, which is used for software testing.

Tuesday, February 25, 2014

Configuring Topolgies in OPNET

Every network element in OPNET is represented as an Object.
Properties of network elements are specified by object attributes.
Properties for these network elements are usually pre-configured with default values.

Basic Attributes represent a single property of the network element, e.g. IP address.
Compound Attributes group basic attributes based on some common characteristics, e.g. TCP Parameters consists several basic attributes such as receive buffer size, MSS, max ACK delay etc.

The network element's pop-up menu allows to change the attributes values.

In Edit Attributes Dialog Box, the Attribute Name column list the compound attributes as tree view.
In edit attributes dialog, checking Apply selected objects checkbox will apply attribute changes to all objects that have been selected.

Some objects may contain multiple instances of the same attribute, e.g. server may support multiple application services, each of which may be configured separately; in such situation an attribute may have a child called Number of Rows.

By default, value of Number of Rows for most attributes is 0.

OPNET uses the model hierarchy. This hierarchy contains levels as:
  • Process Level (Process Editor)
  • Node Level (Node Editor)
  • Network Level (Project Editor)
  • Simulation Level
  • Process Editor enables to develop and compile process models, process model allows to model individual network protocols and technologies.
  • Node Editor defines individual network devices which are modeled as one or more modules; every model has an associated process model which defines the operation of module.
  • Project Editor allows to organize node models/network devices into a network topology and change attributes of the objects in the created topology.
  • Other editors can be Link Editor, Path Editor, Demand Editor, Packet Format Editor etc.
The attribute values can be promoted to the node, to the network, and even to the simulation level.

To promote attribute to higher level, in Edit Attributes dialog right click on value of attribute and Select Promote Attribute to Higher Level.