Question 1

What is CIDR prefix notation and how does IPv4 subnetting work?

Accepted Answer

Classless Inter-Domain Routing (CIDR) specifies how many leading bits in a 32-bit IPv4 address belong to the network prefix (e.g. /24 means 24 network bits and 8 host bits = 256 total IP addresses).

Question 2

Why are two IP addresses reserved in every standard IPv4 subnet?

Accepted Answer

In standard IPv4 subnets (prefix <= 30), the lowest address (all host bits 0) is reserved as the Network Identifier, and the highest address (all host bits 1) is reserved for the Directed Broadcast address, leaving 2^(32-prefix) - 2 usable host addresses.

Question 3

What are RFC 1918 Private IP address ranges and where are they used?

Accepted Answer

RFC 1918 defines non-routable private IP blocks reserved for local home and corporate networks: 10.0.0.0/8 (10.0.0.0–10.255.255.255), 172.16.0.0/12 (172.16.0.0–172.31.255.255), and 192.168.0.0/16 (192.168.0.0–192.168.255.255).

Question 4

What is an RFC 3021 /31 subnet and how does it save IP space on point-to-point links?

Accepted Answer

RFC 3021 permits /31 subnets on point-to-point router links without reserving network or broadcast addresses, allowing both allocated IP addresses to be assigned directly to router interfaces and conserving valuable address space.

Question 5

What is a Wildcard Mask and where is it used?

Accepted Answer

A wildcard mask is the bitwise inverse of a subnet mask (255.255.255.255 minus the subnet mask). For example, a /24 subnet mask 255.255.255.0 has a wildcard mask of 0.0.0.255, widely used in router Access Control Lists (ACLs) and OSPF routing configurations.

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