IPv4
32-bit addresses, dotted quad, fragmentation, and a header you should be able to sketch.
Level: beginner
What is it?
IPv4 is the 32-bit version of Internet Protocol. Addresses are usually written as four decimal octets, such as 192.0.2.10, and CIDR prefixes specify how many leading bits identify a network.
Why does it matter?
IPv4 addressing and subnetting are foundational for routing, DHCP configuration, access-control rules, NAT, and troubleshooting connectivity across LANs and WANs.
Mental model
A dotted-decimal address is simply 32 bits displayed in four groups of eight. A /24 prefix means the first 24 bits form the network prefix and the remaining eight bits identify addresses within that subnet.
How it works
- Each octet represents a value from 0 through 255.
- CIDR notation such as /24 expresses prefix length; a longer prefix leaves fewer host bits.
- For a conventional IPv4 subnet, the all-zero host part identifies the network and the all-one host part is the directed broadcast, with special-purpose exceptions such as /31 point-to-point links.
- The header includes source/destination addresses, protocol, TTL, and fields used for fragmentation and header integrity.
- Private IPv4 ranges are not globally routed as public addresses; NAT is common but does not itself replace firewall policy.
Steps
- Convert 192.0.2.10 to 32 bits: four octets, each eight bits.
- Apply a /24 mask to identify 192.0.2.0 as the network prefix.
- The host part is the final octet for /24.
- Check routing, mask, gateway, and ACLs separately when diagnosing reachability.
Real-world usage
- Calculating network and broadcast addresses.
- Reading interface configurations and firewall rules.
- Understanding the roles and limitations of NAT.
Common mistakes
- Using the first three dotted octets as a universal subnet rule; prefixes can be any supported length.
- Assuming every address ending in .255 is a broadcast address regardless of prefix.
- Confusing private addressing with encryption or security.
Interview questions
How many bits are in an IPv4 address?
32 bits, commonly displayed as four decimal octets.
What does 192.0.2.10/24 mean?
The prefix is 24 bits; the conventional subnet is 192.0.2.0/24 with host addresses in the remaining eight bits, subject to special-address rules.