Question

Consider a router with the IP address 192.168.1.254/26:

a) Indicate the subnet mask for the above address.
b) Demonstrate calculation of the network address (including binary and decimal values).
c) Write the relevant DHCP subnet configuration text for this network where the first 50 addresses will be dynamically allocated to hosts.

Answer 1

 Ans a,b)  An IP address is split into two components a network component and a node component.In early IPv4 networks address classes were used to identify the number of bytes allocated to the network component.The main classes were class A,B,C. With the allocation as follows:

Class A    network,node,node,node
Class B   network,network,node,node
Class C    network,network,network,node

To determine the class you needed to examine the most significant byte (far left).

• 0-127 Class A
• 128-191 Class B
• 192-ccc Class C

It is important to understand that the network part of the address is only used for routing IP packets on the public internet.Once the packet enters the private network then the Node address is used and the public Network address is not used.

Now a network administrator can interpret the node address any way they want, and so it is possible to split the node address into subnet and Node. So we could have

Net.Subnet.Node.Node or Net.Subnet.Subnet.Node.

The technique used to create subnets is to use a Mask.

The mask effectively hides the Node component and leaves the network and subnetwork components.
Address: 192.168.1.254 11000000.10101000.00000001.11 111110 Netmask: 255.255.255.192 = 26 11111111.11111111.11111111.11 000000 Wildcard: 0.0.0.63 00000000.00000000.00000000.00 111111 => Network: 192.168.1.192/26 11000000.10101000.00000001.11 000000 (Class C) Broadcast: 192.168.1.255 11000000.10101000.00000001.11 111111 HostMin: 192.168.1.193 11000000.10101000.00000001.11 000001 HostMax: 192.168.1.254 11000000.10101000.00000001.11 111110

c)

To configure a host, we require the following things:

• Leased IP address – IP address to a host that lasts for a particular duration which goes for a few hours, few days, or few weeks.
• Subnet Mask – The host can know on which network it is on.
• Gateway address – The Gateway is the Internet Service Provider that connects users to the internet. The Gateway address lets the host know where the gateway is to connect to the internet.

DHCP server assigns IP address to a host as:

• DHCPDISCOVER: When a new node is connected to the network, it broadcasts the DHCPDISCOVER message which contains the source address as 0.0.0.0 to every node on the network including the server. DHCP server on receiving the message returns the DHCPOFFER message to the requested host which contains the server address and new IP address to the node.
• DHCPOFFER: If there are multiple servers on the network, the host receives multiple DHCPOFFER messages. It is up to the host to select a particular message.
• DHCPREQUEST:  The requested host on receiving the offer message, it again broadcasts the DHCPREQUEST message on the network with the address of the server whose offer message is accepted by the host. The server which pertains to that server address sent by the host checks whether the address to be assigned to the node is available in the data storage.
• DHCPACK : If the address is assigned, it marks the IP address in the storage as unavailable to ensure consistency. Now, the server sends the DHCPACK packet to the requested host which contains network information(IP address, subnet mask, gateway address). In case, if the address is assigned to another machine meanwhile, then the server sends the packet DHCPNAK to the requested host indicating that the IP address is assigned to some other machine.
• DHCPRELEASE : And finally, If the host wants to move to another network or if it has finished its work, it sends the DHCPRELEASE packet to the server indicating that it wants to disconnect. Then the server marks the IP address as available in the storage so that it can be assigned to other machines.