Semester 3 - Computer Networks - Mock

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Q1. What is the first 24 bits in a MAC address called?

- NIC
- BIA
- OUI
- VAI

Answer: OUI

Explanation: An Organizationally Unique Identifier (OUI) is a 24-bit number that uniquely identifies a vendor, manufacturer, or other organization globally or worldwide. They are used as the first 24 nits of the MAC address to uniquely identify a particular piece of equipment.

Q2. The figure(?) below shows subnetting of class C address with Network Address 192.168.10.0/27. Please note that /27 is the network prefix here.\
Ensure that you create two subnets named subnet 1 and subnet 2 and assign the appropriate addresses along with the network prefix. Answer the following questions

a). Mention the network address of subnet 1 along with network prefix?
b). What is the broadcast address of subnet 1?
c). What is the network address of subnet 2 along with network prefix?
d). What is the broadcast address of subnet 2?
e). Host 0 is the first usable IP address in subnet 1, what is the IP address of Host 0?
f). Default gateway of first subnet is the last usable address, what is the default gateway of subnet 1?
9). Host 1 and Host 2 is the respective first and second usable IP address in subnet 2, What is the IP address of Host1 and Host2.
h). Default gate way of second subnet is the last usable address, what is the default gateway of subnet 2?

Answer.

Basics First:

  • IP4 address is expressed in 4 octets meaning 8x4=32 bits in total. (our IP address is 192.168.10.0)
  • /27 is CIDR notation to show first 27 bits are used for network portion out of 32 and rest 5 bits are used for hosts. 5 bits means 2^5 (or 32) possible addresses. You can also say subnet mask is 255.255.255.224
  • While subnetting (dividing a network in two or more smaller networks) we need more bits to make new networks so we “borrow” bits from host portion.
  • First address is network address, last address is broadcast address and rest are for hosts (devices).

Determine the original subnet mask:

  • Original network address is 192.168.10.0/27.
  • The /27 means that the first 27 bits of the address are used for the network prefix, and the remaining 5 bits are used for host addresses.
  • The subnet mask is 255.255.255.224.

Subnet(or divide) the network:

  • Need to create two subnets, so we will borrow 1 bit from the host portion of the address
  • Host part has 32-27 = 5 bits
  • This will increase the subnet mask from /27 to /28. The new subnet mask is 255.255.255.240.

Calculate the size of each subnet:

  • With the /28 prefix, we have 4 bits left for host addresses.
  • Each subnet will have 2^4 = 16 addresses, including the network and broadcast addresses.

Subnet 1:

Network address: 192.168.10.0/28 (same as the original, but with a /28 instead of /27).
Broadcast address: 192.168.10.15 (the 16th address, counting from 0).
First usable address (Host 0): 192.168.10.1 (the second address).
Last usable address (usually the default gateway): 192.168.10.14 (one less than the broadcast address).

Subnet 2:

Network address: 192.168.10.16/28 (next address after subnet 1’s broadcast).
Broadcast address: 192.168.10.31 (16th address, counting from 16).
First usable address (Host 1): 192.168.10.17 (second address in this subnet).
Second usable address (Host 2): 192.168.10.18.
Last usable address (default gateway): 192.168.10.30.

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