1.7 Calculating IPv4 Subnets and Hosts

Question 1

What is VLSM and why is it used in networking?

Answer 1

Variable-Length Subnet Masking (VLSM) allows different subnet masks on different subnets, helping to efficiently allocate IP space.

Transcript: “we create smaller networks… using a process known as VLSM… you… can build a network that is exactly the right size…” (0:59–1:48)

Question 2

What does a /8 subnet mask represent in decimal and in terms of network/host bits?

Answer 2

It equals 255.0.0.0 and means 8 bits for the network and 24 for the host.

“Classful” addressing

Transcript: “the first 8 bits would be 1… 255.0.0.0… a /8 subnet…” (3:03–3:20)

Question 3

What is the benefit of using powers of 2 when subnetting?

Answer 3

It speeds up calculations of subnets and hosts using 2^subnet bits and 2^host bits - 2.

Transcript: “look at a list of the powers of 2… calculate… 2 to the number of subnet bits… hosts… minus 2” (4:22–5:06)

Question 4

Given 10.1.1.0/24, how many subnets and hosts per subnet can you create?

Answer 4

65,536 subnets and 254 hosts per subnet.

11111111.11111111.11111111.00000000

Default for class A address network = 8 bits, so 10.1.1.0/24 has 16 subnet bits

Transcript: “we have 16 subnet bits… 2 to the power of 16… 8 host bits… 2^8 - 2… 254” (5:17–6:18)

Question 5

For IP 192.168.11.0/26, what are the possible subnets and hosts per subnet?

Answer 5

4 subnets and 62 hosts per subnet.

11111111.11111111.11111111.11000000
Class C network address has 24 bits for the network by default, so this IP address has 2 subnet bits

Transcript: “2 subnet bits… 2^2 = 4… 6 host bits… 2^6 - 2 = 62” (6:36–7:51)

Question 6

For IP 172.16.55.0/21, how many subnets and hosts per subnet are available?

Answer 6

32 subnets and 2,046 hosts per subnet.

11111111.11111111.11111000.00000000
Class B address has 16 network bits by default, so this IP address has 5 subnet bits

Transcript: “five subnet bits… 2^5 = 32… 11 host bits… 2^11 - 2 = 2046” (8:00–9:11)

Question 7

Why is calculating subnets and hosts important in network design?

Answer 7

It allows for efficient routing and avoids relying on one device to know every destination.

Transcript: “functionally impossible to have one device that would know… we create smaller networks…” (0:34–0:54)