Question 1

What is RFC 3021 and why does a /31 subnet have 2 usable host addresses?

Accepted Answer

In traditional IPv4 subnetting (/24 to /30), two addresses are reserved per subnet: the network identifier (all zeros) and the broadcast address (all ones). RFC 3021 defines point-to-point links on /31 subnets where network and broadcast addresses are not required, allowing both IP addresses to be assigned to router interfaces with zero wasted address space.

Question 2

How many usable hosts are available in common CIDR subnet masks?

Accepted Answer

Usable host count equals 2^(32 - prefix) - 2 for prefixes /1 to /30. A /24 provides 254 hosts (256 - 2); /25 provides 126 hosts; /26 provides 62 hosts; /27 provides 30 hosts; /28 provides 14 hosts; /29 provides 6 hosts; and /30 provides 2 hosts. RFC 3021 /31 provides 2 usable hosts, and /32 provides 1 single host route.

Question 3

What are the RFC 1918 private IPv4 address ranges?

Accepted Answer

RFC 1918 reserves three address blocks for private networks: Class A: 10.0.0.0/8 (10.0.0.0 – 10.255.255.255); Class B: 172.16.0.0/12 (172.16.0.0 – 172.31.255.255); and Class C: 192.168.0.0/16 (192.168.0.0 – 192.168.255.255). These addresses cannot be routed across the public Internet.

Question 4

What is a wildcard mask and how is it used in Cisco access lists?

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 (255.255.255.0) has a wildcard mask of 0.0.0.255. Routers use wildcard masks in ACLs and OSPF configurations where a 0 bit means match exactly and a 1 bit means ignore.

Question 5

How does variable-length subnet masking (VLSM) divide a network into smaller subnets?

Accepted Answer

VLSM splits a larger network prefix into subnets of different sizes to match actual host counts without address waste. For instance, a /24 network can be split into two /25 subnets (126 hosts each) or four /26 subnets (62 hosts each) by borrowing additional network bits from the host portion.

Question 6

Does calculating an IP subnet leak my network or IP configuration?

Accepted Answer

No. All bitwise operations, binary breakdowns, usable ranges, and wildcard mask calculations are computed strictly in your browser using unsigned 32-bit integer arithmetic. Zero network requests, telemetry, or external DNS calls are executed.

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