Classless IP Addressing

Classless IP Addressing - Q & A

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Before questions

Slash Notation

Three Rules in CIDR Block

Question 1

Given the CIDR representation 168.1.2.10 / 18. Find the range of IP Addresses in the CIDR block.

IP address - 168.1.2.10

10101000.00000001.00000010.00001010

Number of bit allocated to Network ID - 18

Number of bit allocated to Host ID - 32-18 = 14

Subnet mask - 11111111.11111111.11000000.00000000

First address = (given address) AND (Subnet mask)
	1	0	1	0	1	0	0	0	.	0	0	0	0	0	0	0	1	.	0	0	0	0	0	0	1	0	.	0	0	0	0	1	0	1	0
	AND
	1	1	1	1	1	1	1	1	.	1	1	1	1	1	1	1	1	.	1	1	0	0	0	0	0	0	.	0	0	0	0	0	0	0	0
	1	0	1	0	1	0	0	0	.	0	0	0	0	0	0	0	1	.	0	0	0	0	0	0	0	0	.	0	0	0	0	0	0	0	0
				1	6	8			.				1					.				0					.				0

Last Address = (any address) OR [NOT (Subnet mask)]

	NOT (Subnet mask)						=
		1	1	1	1	1	1	1	1	.	1	1	1	1	1	1	1	1	.	1	1	0	0	0	0	0	0	.	0	0	0	0	0	0	0	0
		0	0	0	0	0	0	0	0	.	0	0	0	0	0	0	0	0	.	0	0	1	1	1	1	1	1	.	1	1	1	1	1	1	1	1

Last Address = (any address) OR [NOT (Subnet mask)]
		1	0	1	0	1	0	0	0	.	0	0	0	0	0	0	0	1	.	0	0	0	0	0	0	1	0	.	0	0	0	0	1	0	1	0
																																		OR
		0	0	0	0	0	0	0	0	.	0	0	0	0	0	0	0	0	.	0	0	1	1	1	1	1	1	.	1	1	1	1	1	1	1	1
		1	0	1	0	1	0	0	0	.	0	0	0	0	0	0	0	1	.	0	0	1	1	1	1	1	1	.	1	1	1	1	1	1	1	1
					1	6	8			.				1					.				6	3				.				2	5	5

Range of IP Addresses = 168.1.0.0 - 168.1.63.255

Question 2

Consider a block of IP Addresses ranging from 100.1.2.32 to 100.1.2.47.

1. Is it a CIDR block?

2. If yes, give the CIDR representation.

1. If it is a CIDR block, 3 rules must be satisfied.

Rule 01 - All the IP Addresses must be contiguous unallocated addresses.

Clearly, all IP addresses are contiguous.

Rule 01 - Correct

Rule 02 - Size of the block must be power of 2

Number of IP address in block = 47 - 32 + 1

Size of the block = 16

Size of the block = 2^4

Rule 02 - Correct

Rule 03 - The first address of every block must be divisible by the length of the block.

First IP address - 100.1.2.32 - 01100100.00000001.00000010.00100000

Last 4 bits (4 least significant bits) are Zero.

That means this address divisible by 2^4

Rule 03 - Correct

So, this IP range / Block is a CIDR Block

Size of Block - Total number of IP address = 16 = 2^4

Length of Host ID = 4

Length of Network ID = 32 - 4 = 28

CIDR representation - 100.1.2.32/28

Question 3

Consider a block of IP Addresses ranging from 150.10.20.64 to 150.10.20.127.

Is it a CIDR block?
If yes, give the CIDR representation.

1. If it is a CIDR block, 3 rules must be satisfied.

Rule 01 - All the IP Addresses must be contiguous unallocated addresses.

Clearly, all IP addresses are contiguous.

Rule 01 - Correct

Rule 02 - Size of the block must be power of 2

Number of IP address in block = 127 - 64 + 1

Size of the block = 64

Size of the block = 2^6

Rule 02 - Correct

Rule 03 - The first address of every block must be divisible by the length of the block.

First IP address - 150.10.20.64 - 10010110.00001010.00010100.01000000

Last 6 bits (6 least significant bits) are Zero.

That means this address divisible by 2^6

Rule 03 - Correct

So, this IP range / Block is a CIDR Block

Size of Block - Total number of IP address = 16 = 2^6

Length of Host ID = 6

Length of Network ID = 32 - 6 = 26

CIDR representation - 150.10.20.64/26

Question 4

Perform CIDR aggregation on the following IP Addresses-

200.96.86.0/24

200.96.87.0/24

200.96.88.0/24

200.96.89.0/24

Rule 01 - All the IP Addresses must be contiguous unallocated addresses.

Clearly, all IP addresses are contiguous.

Rule 01 - Correct

Rule 02 - Size of the block must be the power of 2

Each IP address prefix length is = 24

Suffix length is = 32 -24 = 8

Total number of IP Addresses = 28 + 28 + 28 + 28 = 4 x 2^8 = 2^2 x 2^8 = 2^10.

Rule 02 - Correct

Rule 03 - The first address of every block must be divisible by the length of the block.

First IP address - 200.96.86.0 - 11001000.01100000.01010110.00000000

Last 9 bits (9 least significant bits) are Zero.

That means this address not divisible by 2^10

Rule 03 - Incorrect

All 3 rules are not satisfied. Because of that cant Perform CIDR aggregation

Question 5

Two computers C1 and C2 are configured as follows. C1 has IP address 203.197.2.53 and netmask 255.255.128.0. C2 has IP address 203.197.75.201 and netmask 255.255.192.0. which one of the following statements is true?

(A) C1 and C2 both assume they are on the same network

(B) C2 assumes C1 is on same network, but C1 assumes C2 is on a different network

(C) C1 assumes C2 is on same network, but C2 assumes C1 is on a different network

(D) C1 and C2 both assume they are on different networks.

For more clear let's write all the IP addresses and Subnet masks in binary format.

C1 IP - 203.197.2.53 - 11001011.11000101.00000010.00110101

C2 IP - 203.197.75.201 - 11001011.11000101.01001011.11001001

C1 subnet mask - 255.255.128.0 - 11111111.11111111.10000000.00000000

C2 subnet mask - 255.255.192.0 - 11111111.11111111.11000000.00000000

Let see now look at the network ID that belongs to the C1 IP address Block.

C1 Network ID = (given address) AND (Subnet mask)
	1	1	0	0	1	0	1	1	.	1	1	0	0	0	1	0	1	.	0	0	0	0	0	0	1	0	.	0	0	1	1	0	1	0	1
	AND
	1	1	1	1	1	1	1	1	.	1	1	1	1	1	1	1	1	.	1	0	0	0	0	0	0	0	.	0	0	0	0	0	0	0	0
	1	1	0	0	1	0	1	1	.	1	1	0	0	0	1	0	1	.	0	0	0	0	0	0	0	0	.	0	0	0	0	0	0	0	0
				2	0	3			.				1	9	7			.				0					.				0

Now C1 see C2 network ID and subnet mask of C1.

= (given address) AND (Subnet mask)
	1	1	0	0	1	0	1	1	.	1	1	0	0	0	1	0	1	.	0	1	0	0	1	0	1	1	.	1	1	0	0	1	0	0	1
	AND
	1	1	1	1	1	1	1	1	.	1	1	1	1	1	1	1	1	.	1	0	0	0	0	0	0	0	.	0	0	0	0	0	0	0	0
	1	1	0	0	1	0	1	1	.	1	1	0	0	0	1	0	1	.	0	0	0	0	0	0	0	0	.	0	0	0	0	0	0	0	0
				2	0	3			.				1	9	7			.				0					.				0

Now you can see C1 and C2 IP address as the same network ID

Let see now look at the network ID that belongs to the C2 IP address Block.

C2 Network ID = (given address) AND (Subnet mask)
	1	1	0	0	1	0	1	1	.	1	1	0	0	0	1	0	1	.	0	1	0	0	1	0	1	1	.	1	1	0	0	1	0	0	1
	AND
	1	1	1	1	1	1	1	1	.	1	1	1	1	1	1	1	1	.	1	1	0	0	0	0	0	0	.	0	0	0	0	0	0	0	0
	1	1	0	0	1	0	1	1	.	1	1	0	0	0	1	0	1	.	0	1	0	0	0	0	0	0	.	0	0	0	0	0	0	0	0
				2	0	3			.				1					.				6	4				.				0

Now C2 see C1 network ID and subnet mask of C2

= (given address) AND (Subnet mask)
	1	1	0	0	1	0	1	1	.	1	1	0	0	0	1	0	1	.	0	0	0	0	0	0	1	0	.	0	0	1	1	0	1	0	1
	AND
	1	1	1	1	1	1	1	1	.	1	1	1	1	1	1	1	1	.	1	1	0	0	0	0	0	0	.	0	0	0	0	0	0	0	0
	1	1	0	0	1	0	1	1	.	1	1	0	0	0	1	0	1	.	0	0	0	0	0	0	0	0	.	0	0	0	0	0	0	0	0
				2	0	3			.				1	9	7			.				0					.				0

The network ID of the C2 is different.

So answer -

(C) C1 assumes C2 is on the same network, but C2 assumes C1 is on a different network.

Chathuni Thilakarathne

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