IPv4 Subnet Calculator

An IPv4 address is a 32-bit number. We write it in the friendly "dotted-quad" form (192.168.1.42) but underneath it is just four bytes packed in a row. The netmask — also 32 bits — is what tells the routing layer which part of those bits is the network identifier and which part is the host identifier. The CIDR notation /24 means "the first 24 bits are network, the last 8 bits are host" — equivalent to a netmask of 255.255.255.0. Every subnet calculation is a deterministic application of bitwise AND, OR and NOT against that mask.

Why CIDR replaced classes

Before 1993, IPv4 was split into rigid "classes" — Class A used the first byte for the network (16 million hosts each), Class B used two bytes, Class C used three. The problem was waste: a company that needed 500 IP addresses was given a full Class B (65,534 hosts) because Class C maxed out at 254. CIDR — Classless Inter-Domain Routing — broke the rigid boundaries and let networks be sized to any power of two. A /22 carved out 1,022 usable hosts, exactly fitting that 500-user company without the wasted 65,000 addresses. CIDR is why IPv4 has survived as long as it has.

The three private ranges and CGNAT

RFC 1918 reserves three IPv4 ranges for private use — addresses that won't be routed on the public internet. They are 10.0.0.0/8 (the giant one, used by most corporate networks), 172.16.0.0/12 (Docker's default for many setups), and 192.168.0.0/16 (home routers). In 2012, RFC 6598 added a fourth range — 100.64.0.0/10 — specifically for Carrier-Grade NAT (CGNAT), where ISPs share one public IP among many subscribers. If your Singapore or Malaysian home broadband shows an IP starting with 100.64-127, you are behind CGNAT — your ISP didn't give you a dedicated public IP, and incoming connections won't reach you without a relay or IPv6.

Singapore and APNIC allocations

The five regional internet registries (RIRs) allocate IPv4 blocks to ISPs. Singapore, Malaysia, Indonesia, Thailand, and the rest of Asia-Pacific are served by APNIC (apnic.net), headquartered in Brisbane. APNIC's allocations to Singapore-based ISPs typically come from the 203.116.0.0/16, 202.157.0.0/16, and 118.200.0.0/16 ranges, among others. When you whois a Singapore IP, the netname will often start with SG- and the org will be SingTel, StarHub, M1, or a smaller ISP. APNIC ran out of fresh /8 allocations from IANA in 2011; new allocations to ASEAN ISPs since then have come from recycled space or IPv6.

Why the broadcast and network can't be used

In a /24 like 192.168.1.0/24, the addresses .0 and .255 are reserved. The all-zeros host part (.0) is the network identifier — used by routers to refer to the subnet itself. The all-ones host part (.255) is the broadcast address — packets sent there are delivered to every host on the subnet. That's why a /24 has 256 total addresses but only 254 usable host addresses. Point-to-point /31 links and modern host-routes (/32) are the exceptions where this convention is relaxed by RFC 3021 — useful trivia for network engineers but not relevant for typical home or office subnets.

Picking the right prefix length

The rule of thumb: count hosts, round up to the next power of two, then add two for network and broadcast. A team of 50 servers? Round to 64, that's a /26 (64 addresses, 62 usable). A new VPC on AWS for a startup? /16 gives you 65,534 addresses — overkill, but cheap to provision and lets you sub-divide later. Avoid the temptation to be parsimonious early; the cost of re-IP'ing later is hours of downtime, the cost of provisioning a /16 today is zero.