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.

Friday, February 21, 2014

Random Points on Topologies in OPNET

Physical topology is the actual layout of the nodes and links.
Logical topology deals with protocol configuration of the network devices for the establishing channels.
Physical topologies include fully-connected mesh, partially-connected mesh, bus, star, ring, ring and hybrid.

The Object Palette provides access to OPNET models. You can access it through Topology->Object Palette 
  • Node Models consists of hubs, switches, routers etc.
  • Link Models contains models of links e.g. 10BASE-T.
  • Path Models contains model for specifying network paths e.g. Multiprotocol Label Switching (MPLS).
  • Demand Models specify the data flows, IP voice traffic, IP security etc.
  • Wireless Domain models, contains sparse grid, mobility etc.
  • Shared Objects contain models group according to common properties e.g. internet_toolbox group contains node, link etc. for modeling the Internet.
Model naming follows convention like companyabrivation_model_slots_ethernetinterfaces_framerelayinterfaces_...

OPNET also provides some models of complete networks e.g. 10BaseT_LAN models a LAN in which nodes are connected via 10BASE_T Ethernet links.
Nodes like Application Config, IP VPN Config, Profile Config etc. don't represent real-life devices but allows the configuration of various applications and technologies.
ppp_wkstn  and  ethernet_wkstn  are  used  to  model  client  workstations.
ppp_server and ethernet_server are used to model servers.
1000BaseX_LAN, 100BaseT_LAN, and  10BaseT_LAN  models  are  used to simulate Ethernet LANs. By default, each of the above network models simulates the operation of a LAN with 10 end nodes.
ethernet4_slip8_gtwy models a gateway router.
ethernet2_slip8_firewall also models a gateway router but with additional firewall features.
ip32_cloud models an IP cloud and is commonly used to represent the connectivity  of  the  Internet
PPP_28K, PPP_33K, PPP_56K, PPP_DS1, and PPP_DS3 are models of duplex point-to-point links with various data rates.

You can also create custom model by selecting Create Custom Model button in Object Pallet.

Rapid Configuration is a useful tool for creating network topologies with a large number of nodes. You can open it by selecting select Topology > Rapid Configuration…

Select Topology->Verify Links to verify link conectivity.

Select Topology->Fail Selected Objects to fail the selected object during a specific point in simulation and then recover it in other point of time.

With the help of the subnet object, you can arrange nodes in the network in a hierarchical fashion.

In fixed subnet nodes are not allowed to move during the simulation.

Double-click on the subnet icon will take you inside the subnet.

Wednesday, February 19, 2014

Random Points about OPNET

OPNET is the network simulation software.
Compare to OPNET IT Guru, OPNET Modeler has additional functionalities.

When you install OPNET create (C:\Users\user_name\op_models) three folders to store user-specific information:
  • op_admin, contains backup, temp and log files etc.
  • op_models, stores project files.
  • op_reports, stores simulation study reports.

OPNET saves preferences in op_admin directory with file named env_db<version>.

To change the default model directory follow path File->Manage Model Files ->Add Model Directory
Each project is divided into one or more scenarios. A scenario is a simulation study of particular system or technology under different configuration settings.

E.g. a project may contain scenarios like 5nodes_10K, 50nodes_100K etc. Scenario also helps to organize simulation studies into small parts that examine a specific aspect or configuration.

The Startup Wizard allows the creation of projects.
To create new scenario, select Scenarios->New Scenario
The Scenarios->Scenario Components allows importing or exporting of various components like mode, probe model etc.

Import operation overwrites existing components of a scenario.

Sunday, February 16, 2014

Design Stages of IPv4 Subnetting

The design process of subnetting may involve the following steps:
  1. Investigate the Requirements for Subnetting
  2. Design your subnets to meet your requirements
  3. Decide how to implement the subnets

1) Investigate the Requirements for Subnetting

In investigation stage you decide:
  • Which nodes or hosts should be grouped in the same subnet?
  • Next by studying network diagrams etc. decide the total number of subnets, keep in mind that VLANs also require separate subnets to operate.
  • Decide the number of IP addresses each subnet will require, this will also include IP addresses for the router interfaces and switch IP addresses which are to be used for telnet, ssh etc.
  • Decide to use a single subnet mask for all subnets or variable length subnet mask (VLSM) to avoid wastage of IP addresses.

2) Design your Subnets to Meet your Requirements

At design stage you will:
  • Select the Internet Addressing Architecture
  • Select the Subnet Mask
  • Make list of all subnets to use

Internet Addressing Architecture
Internet Addressing Architecture divides the IP addresses into to two address spaces.
Private IP Addresses: Packets addresses with private IP address cannot be routed or transmitted through the Internet, however; they can be used by enterprises within their local networks. Following is the list of private IP addresses:
Class A --- (No. of networks 1)
Class B --- through (No. of networks 16)
Class C – through (No. of networks 256)
Public IP Addresses: The Internet design requires that any one connecting to the Internet has to use a Public IP address, public IP addresses can only be routed through Internet. The problem face the IPv4 public IP address space is that it exhausted in 2011, to solve the issue the Internet community come-up with solutions like:
  • 128-bit IPv6
  • Network Address Translation (NAT); which allows the mapping of hundreds of private IP addresses to a single or more than one public IP address

Select the Subnet Mask
Once you decided on the number of subnets, host per subnet required, type of subnet and choose the Internet addressing architecture, next step would be to select the mask. Subnet mask define the network, subnet and host part of the network.

Mask selected must have enough subnet bits to number all subnets and at the same time the host part bits must also be large enough to accommodate the number of hosts in each subnet.

It’s worth mentioning here:
  • That the difference between Classful and Classless Networks is that Classful network use the default mask define for the Class (e.g. Class A’s default mask is, while the Classless network use the variable length subnet mask.
  • Subnet mask can be represented in decimal (, binary (11111111.00000000.00000000.00000000), or prefix (/8)

Make List of All Subnets to Use
Find the subnet ID, subnet broadcast for each subnet, and IP addresses to use.
Subnet ID is the numerically the smallest number in subnet and it can’t be used as IP address for host.
Subnet Broadcast address is the numerically highest number in subnet and it is used for directed broadcast in the subnet.
IP addresses are the addresses between the subnet ID and subnet broadcast address, it can be used as host’s IP address.

3) Decide How to Implement the Subnets

Base on your network diagram identify the location for subnet, pick one from the list. Separate the static IP addresses and dynamic IP addresses assigned through DHCP.