---
{
  "title": "IPv4",
  "summary": "Read IPv4 addresses with their actual prefixes, allocation context, and translation history rather than relying on historical address classes.",
  "topic": "networking",
  "level": "Beginner",
  "updated": "2026-10-12",
  "units": [
    1
  ],
  "aliases": [],
  "sources": [
    {
      "title": "Cisco: Networking basics",
      "url": "https://www.cisco.com/site/us/en/learn/topics/networking/what-is-computer-networking.html"
    },
    {
      "title": "IETF: IPv4 specification",
      "url": "https://www.rfc-editor.org/rfc/rfc791.html"
    },
    {
      "title": "IETF: CIDR",
      "url": "https://www.rfc-editor.org/rfc/rfc4632.html"
    }
  ],
  "published": "2026-10-02",
  "editorialVersion": 2
}
---
## The idea

IPv4 uses 32-bit addresses, commonly shown as four decimal octets. Prefixes describe networks, and routing chooses paths between them. Public, private, loopback, multicast, and documentation ranges serve different purposes. Modern networks use CIDR rather than relying on historical classful assumptions. An address’s meaning depends on the network, time, assignment, and possible translation.

## In practice

A lab uses a private subnet, while reports use documentation addresses to avoid referring to real unrelated systems.

## A common misunderstanding

Historical Class A, B, and C labels do not determine every modern network’s actual prefix.

## Check your understanding

What should you use to determine the local network size?

**Think it through:** The configured prefix length or subnet mask, not only the address’s first octet.

## Read the address and prefix together

Imagine a fictional training lab uses 192.168.40.0/24. The dotted address notation is familiar, but the prefix is what defines this particular block. Changing the prefix changes which destinations a correctly configured host considers local. Historical classroom labels cannot substitute for the configured boundary.

A report should state whether an address is a host observation, a subnet, or a translated endpoint. Writing just “192.168.40” leaves both the scope and the exact meaning unclear. Include the relevant interface, network, and time when describing an operational observation.

## Special ranges have specific purposes

Private addressing supports reuse inside different networks; it does not identify one global organization. Loopback and documentation addressing serve other purposes. Before using a range in a lab or published example, consult its intended allocation rather than choosing a real public address arbitrarily.

For the fictional academy, a diagram can use private internal ranges and clearly labeled invented endpoints. It should not suggest that every publicly formatted address belongs to an actual service the learner may test. Authorized scope is separate from address syntax.

## A packet's address is evidence with limits

Suppose an analyst finds an IPv4 source in a connection record. Address assignment, translation, shared gateways, and the record's collection point affect its interpretation. The external address seen by a service can differ from the address configured on the originating laptop. It also may not identify the person operating that laptop.

For a practical exercise, write a local address with its prefix and identify a local peer and an off-link destination. Explain the next-hop decision without sending traffic to unrelated systems. Then annotate which address a remote observer would see if the gateway translates it. Distinguish the arithmetic, the forwarding decision, and the identity claim; they answer different questions.

## Keep exploring

- [[ip-addressing]]
- [[subnetting]]
- [[nat]]