Subnetting

How many bits are in an IPv4 address?

32 bits, assigned as a unique value to each host a on a network segment.

How are IPv4 addresses commonly written for humans?

In dotted decimal notation.

How many bits are in each dotted-decimal (octet)

8 bits.

What do we call the four groupings in a dotted-decimal IP address?

Octets.

In an IPv4 address, what determines where the network ID ends and host ID begins?

The subnet mask.

What are the three approaches to IPv4 addressing/subnetting?

Classful addressing, Fixed-Length Subnet Masks (FLSM), and Non-Fixed-Length Subnet Masks (VLSM/CIDR).

In classful addressing, what are the common default masks for Classes A,B, and C?

Class A: /8, Class B: /16, Class C: /24

What hierarchy depth does the classful model represent?

A 2-layer hierarchy: network and host.

What did FLSM introduce conceptually?

The idea of sub-networks using a fixed-length mask across all subnets.

What hierarchy depth does FLSM create?

A 3-Layer hierarchy: major network, subnet, host.

What does VLSM allow that FLSM does not?

Different subnet masks per subnet, enabling variable sizes tailored to need.

What is CIDR

Classless Inter-Domain Routing

How does CIDR help routing scalability?

It groups networks into larger blocks to shrink the size of routing tables.

Contrast FLSM vs VLSM.

FLSM uses one mask for all subnets; VLSM uses different masks per subnet.

In class-based IPv4, one portion identifies the network and the other identifies what?

The host.

What are the fixed leading bit patterns (Class IDs) for Classes A, B, and C?

Class A: 0; Class B: 10; Class C: 110.

What does the “Class ID” segment mean in classful addressing?

A fixed leading bit pattern that determines the class and split between network and host.

For Class A, how many bits are used for the network ID and host ID?

Network: 7 bits, Host: 24 bits.

How many different network IDs and host IDs are available in Class A?

126 network IDs; 16,777,214 host IDs per network.

For Class B, how many bits are used for the network ID and host ID?

Network: 14 bits, Host: 16 bits.

How many different network IDs and host IDs are available in Class B?

16,382 network IDs; 65,534 host IDs per network.

For Class C, how many bits are used for the network ID and host ID?

Network: 21 bits, Host: 8 bits.

How many different network IDs and host IDs are available in Class C?

2,097,150 network IDs; 254 host IDs per network.

What is a subnetwork (subnet)?

A smaller network segment carved out of a larger “major” network.

What creates a subnetwork from a major network?

Applying a subnetwork mask (subnet mask).

How many bits long is a subnet mask in IPv4?

32 bits.

What operation does the subnet mask perform with an IP address to find the Network ID?

A logical AND.

In classful subnetting, how many times can you subnet the major network?

Only once.

In classful subnetting, must all subnets be the same size?

Yes, all subnets must be equivalent size.

What two subnet IDs are reserved in classful subnetting?

The all-zeros subnet ID (original network) and the all-ones subnet ID (broadcast).

Which host ID pattern represents the subnetwork ID?

All 0s in the host field.

What does VLSM allow that fixed-length subnetting does not?

Different masks per subnet for granular address allocation.

How does VLSM help manage address space?

It provides more granular control, matching subnet sizes to actual needs.

What does CIDR ignore that classful addressing relied on?

Traditional address classes (A/B/C).

What big advantage does CIDR provide?

The most granular control of address space and efficient route aggregation.

What concept did CIDR popularize for routing efficiency?

Supernetting (aggregating multiple networks under one prefix).What does subnetting require to create a more specific network ID?

What does subnetting require to create a more specific network ID?

Borrowing bits from the host portion to form a subnet ID.

Original network example: what does a /16 mask look like in binary?

11111111 11111111 00000000 00000000.

After borrowing 8 bits from a /16, what is the new mask?

/24, i.e., 11111111 11111111 11111111 00000000.

If you borrow 8 bits, how many subnetworks are created?

2^8 = 256 subnetworks.

How are those 256 subnets uniquely identified?

By the 8-bit subnet ID values (0–255) in the borrowed bit field.

What happens to the host portion of the subnet mask when creating a subnet?

It is divided into borrowed subnet bits and remaining host bits.

What do the borrowed bits become?

Part of the network portion forming a new network number.

What must change when subnets are created?

The subnet mask, and it applies to all resulting subnets.

How many host bits must remain?

At least 2.

Usable Hosts per Subnet formula?

2^(remaining host bits) − 2

Number of Subnets created formula?

2^(borrowed bits)

What are the two “needs” to calculate before applying a mask?

Required number of subnets and required number of hosts.

What operation reveals the network ID from an IP address?

Logical AND with the subnet mask.

What are the three Private IP blocks?

10.0.0.0/8, 172.16.0.0/12, 192.168.0.0/16

Are all-zeros and all-ones host addresses usable?

No, they are reserved (network & broadcast).