Question 1

What is CIDR notation?

Accepted Answer

CIDR (Classless Inter-Domain Routing) notation appends a slash and a number to an IP address — like 192.168.1.0/24 — to state how many of the address's 32 bits are fixed as the network portion. The remaining bits identify individual hosts inside that network. A /24 fixes the first 24 bits (three full octets), leaving 8 bits — 256 addresses — for hosts, which is why /24 is the default size for a typical home or small-office LAN. Replacing the old rigid Class A/B/C system with an arbitrary bit count is what makes CIDR "classless": a network can be sized to exactly what's needed, from a two-address /31 link up to a sprawling /8.

Question 2

Why are 2 IP addresses reserved in every subnet?

Accepted Answer

Every subnet block reserves its very first address as the network address, which identifies the subnet itself rather than any single device, and its very last address as the broadcast address, which every host on that segment listens to for messages meant for the whole subnet. Neither can be assigned to a host without breaking routing or broadcast delivery, so a /24 with 256 total addresses yields only 254 usable ones. The two exceptions are /31 links, which repurpose both addresses for point-to-point use per RFC 3021 since there's no room for a separate broadcast address, and /32, which is a single host route with no network or broadcast concept at all.

Question 3

What are the private IP address ranges?

Accepted Answer

RFC 1918 sets aside three blocks for private networks that never get routed on the public internet: 10.0.0.0/8 (about 16.7 million addresses, common in large corporate networks), 172.16.0.0/12 (roughly 1 million addresses, spanning 172.16.0.0 through 172.31.255.255), and 192.168.0.0/16 (about 65,000 addresses, the default range for most home routers). Devices inside these ranges reach the internet through NAT on a router or firewall, which swaps the private source address for a single public one. Outside of these, 127.0.0.0/8 is reserved for loopback traffic back to the same machine, and 169.254.0.0/16 is the link-local range a device assigns itself when DHCP fails.

Question 4

What is the difference between /24 and /25?

Accepted Answer

A /24 fixes 24 bits for the network and leaves 8 for hosts, giving 256 total addresses and 254 usable ones. Moving to /25 borrows one more bit for the network, cutting the host portion to 7 bits — 128 total addresses and 126 usable — and splitting the original /24 into two equal /25 halves (for example, 192.168.1.0/25 and 192.168.1.128/25). Every extra bit taken from the host side halves the subnet's usable capacity, which is exactly the trade-off the Subnet Splitter tab above lets you explore: fewer host bits means smaller, more numerous subnets carved from the same address block.

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