Question 1

How many hosts are in a /24 subnet?

Accepted Answer

A /24 subnet has 256 total addresses (2^8) but only 254 usable host addresses. The first address is reserved as the network address and the last as the broadcast address.

Question 2

What is CIDR notation?

Accepted Answer

CIDR (Classless Inter-Domain Routing) notation appends a prefix length to an IP address (e.g., 192.168.1.0/24). The prefix length indicates how many bits are used for the network portion, with the remaining bits for host addresses. Defined in RFC 4632, CIDR replaced the legacy classful system in 1993.

Question 3

How do I calculate a subnet mask from a CIDR prefix?

Accepted Answer

Set the first N bits to 1 and the remaining (32-N) bits to 0, then convert each octet to decimal. For example, /24 = 11111111.11111111.11111111.00000000 = 255.255.255.0. For /22, it is 11111111.11111111.11111100.00000000 = 255.255.252.0.

Question 4

What is a wildcard mask?

Accepted Answer

A wildcard mask is the bitwise inverse of the subnet mask. For a /24 subnet (255.255.255.0), the wildcard mask is 0.0.0.255. Wildcard masks are used in Cisco ACLs, OSPF area network statements, and route-map conditions. Unlike subnet masks, wildcard masks need not be contiguous.

Question 5

What is the difference between a network address and a broadcast address?

Accepted Answer

The network address is the first address in a subnet (all host bits set to 0) and identifies the subnet itself in routing tables. The broadcast address is the last address (all host bits set to 1) and is used to send packets to all hosts in the subnet. Neither can be assigned to a device.

Question 6

What is VLSM and why is it important?

Accepted Answer

VLSM (Variable Length Subnet Masking, RFC 1812) allows different subnets within the same network to use different prefix lengths. This avoids wasting addresses — for example, a point-to-point link only needs a /30 (2 hosts), while a LAN might need a /24 (254 hosts). Plan from largest to smallest, aligning each subnet on its natural prefix boundary.

Question 7

Can I have a /31 or /32 subnet?

Accepted Answer

Yes. /31 (RFC 3021) has 2 addresses and is widely used for point-to-point WAN links — both addresses are usable as host addresses since broadcast is unnecessary on a P2P link. /32 represents a single host route, common for loopback interfaces, host routes in OSPF/BGP, and route-server announcements.

Question 8

What is the difference between subnet mask and wildcard mask?

Accepted Answer

They are bitwise inverses. Subnet mask 255.255.255.0 corresponds to wildcard 0.0.0.255. Subnet masks describe the network/host boundary; wildcard masks describe match patterns in Cisco ACLs and OSPF network statements. Most other vendors (Juniper, Arista, Linux iproute2) use prefix length or subnet mask directly.

Question 9

Are 192.168.x.x and 10.x.x.x the same?

Accepted Answer

Both are private (RFC 1918) but different in size. 192.168.0.0/16 has 65,536 addresses, ideal for home and small offices. 10.0.0.0/8 has 16,777,216 addresses, used by enterprises with many sites. There's also 172.16.0.0/12 with about 1 million addresses (default for Docker bridge).

Question 10

Why does AWS show fewer usable hosts than my calculator?

Accepted Answer

AWS reserves the first four addresses (network, VPC router, DNS, future use) plus the broadcast of every subnet — so a /28 has 11 usable hosts instead of 14. Azure reserves three addresses plus broadcast (also 11 usable for /28). GCP reserves two addresses plus the second-to-last and broadcast (12 usable for /28). Plan with each provider's reservation rules.

Question 11

What is supernetting?

Accepted Answer

Supernetting is the inverse of subnetting — combining contiguous smaller subnets into a larger one for route aggregation. Two adjacent /25s within a /24 boundary can be summarized as a single /24 in routing tables, reducing route table size. CIDR enables supernetting by decoupling prefix length from the legacy classful boundaries.

Question 12

How do I subnet without binary math?

Accepted Answer

Use the 'magic number' trick: 256 minus the relevant subnet mask octet equals the block size. For mask 255.255.255.224 (/27), the magic number is 256 − 224 = 32 — subnets advance in steps of 32: .0, .32, .64, .96, .128, .160, .192, .224. Faster than binary for live troubleshooting and exam scenarios.

Question 13

Can subnets overlap?

Accepted Answer

They should not in normal routing. If two assigned subnets overlap (e.g., 10.0.0.0/16 and 10.0.1.0/24), routers prefer the more specific (longer-prefix) route per longest-prefix-match. In production, overlap usually indicates a planning error and breaks connectivity. Always verify subnets in your IPAM or with this calculator before allocating.

Question 14

What is a /0 subnet?

Accepted Answer

0.0.0.0/0 represents 'all IPv4 addresses' and is used as the default route in routing tables — directing all unmatched traffic to a gateway. It is not assigned to a host. The IPv6 equivalent is ::/0.

Question 15

Why is the broadcast address sometimes called a directed broadcast?

Accepted Answer

Subnet-directed broadcast (e.g., 192.168.1.255 for a /24) sends to every host in a specific subnet and is distinct from the limited broadcast 255.255.255.255, which is link-local only. RFC 2644 disabled IP-directed broadcast forwarding by default to mitigate Smurf-style amplification attacks.

Question 16

How do I subnet IPv6?

Accepted Answer

IPv6 standard host subnets are always /64 (RFC 4291). ISPs typically delegate /48 (65,536 subnets) or /56 (256 subnets) to customers. You subnet by extending the prefix from /48 toward /64. Don't go longer than /64 except for point-to-point links — RFC 6164 allows /127 for inter-router links.

Question 17

What is the difference between /24 and 255.255.255.0?

Accepted Answer

They are equivalent — two notations for the same subnet mask. /24 (CIDR) means 24 bits set to 1: 11111111.11111111.11111111.00000000 = 255.255.255.0 (dotted decimal). CIDR notation is preferred in modern documentation, RFC text, and most router show commands.

Question 18

Is this subnet calculator accurate for CCNA and Network+ exams?

Accepted Answer

Yes. The calculator uses RFC-compliant formulas (RFC 4632 for CIDR, RFC 3021 for /31, RFC 4291 for IPv6 references). Use the binary breakdown view to verify your manual calculations during exam prep, and the CIDR cheat sheet for memorizing common subnet sizes.

Subnet Calculator

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Subnet Calculator

Subnet calculator

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