Classless Addressing (CIDR) Flashcards
What does the abbreviation CIDR stand for?
Classless Inter-Domain Routing
What is the equation for calculating the number of bits to borrow for subnetting (Priority is on the number of networks)?
Number of network = 2**n
where “n” is the number of bits to borrow
What is the equation for calculating the number of bits to borrow for subnetting (Priority is on the number of hosts per network)?
Number of host = (2**n) - 2
where “n” is the number of bits to use for the host portion.
Once you know the “n”, then calculate number of bits to borrow as below:
For Class A = 24 - n
For Class B = 16 - n
For Class C = 8 - n
For a Class C network, if we borrowed one (1) bit, how many subnets will you have?
2
Given all zero network is permitted
For a Class C network, if we borrowed two (2) bits, how many subnets will you have?
4
Calculation: 2**2 = 4
For a Class C network, if we borrowed three (3) bits, how many subnets will you have?
8
Calculation: 2**3 = 8
For a Class C network, if we borrowed four (4) bits, how many subnets will you have?
16
Calculation: 2**4 = 16
For a Class C network, if we borrowed five (5) bits, how many subnets will you have?
32
Calculation: 2**5 = 32
For a Class C network, if we borrowed six (6) bits, how many subnets will you have?
64
Calculation: 2**6 = 64
For a Class C network, if we borrowed seven (7) bit, how many subnets will you have?
Invalid borrowing since this only leaves one (1) bit for the host portion.
Explanation:
- All zeroes on the host portion means it is a network address
- All ones on the host portion means it is a broadcast address
For a Class C network, if we borrowed eight (8) bits, how many subnets will you have?
Invalid borrowing since this leaves no bits for the host portion.
For a Class C network, if we borrowed one (1) bit for subnet, how many hosts will you have?
126
Calculation: Borrowed 1 bit, so 7 bits left for host.
2**7 - 2 = 128 - 2 = 126
For a Class C network, if we borrowed two (2) bits for subnet, how many hosts will you have?
62
Calculation: Borrowed 2 bits, so 6 bits left for host.
2**6 - 2 = 64 - 2 = 62
For a Class C network, if we borrowed three (3) bits for subnet, how many hosts will you have?
30
Calculation: Borrowed 3 bits, so 5 bits left for host.
2**5 - 2 = 32 - 2 = 30
For a Class C network, if we borrowed four (4) bits for subnet, how many hosts will you have?
14
Calculation: Borrowed 4 bits, so 4 bits left for host.
2**4 - 2 = 16 - 2 = 14