Lesson 3: IPv4, IPv6, Gateways, and Addressing Modes

Core path: 42 minutes

An IP configuration is a set of related values, not one number copied from a screen.

IPv4

A typical IPv4 endpoint needs:

IPv4 address
subnet mask / prefix
normally a default gateway for off-subnet traffic
DNS configuration when names are used

Private IPv4 ranges include 10.0.0.0/8, 172.16.0.0/12, and 192.168.0.0/16. Public IPv4 addresses are globally routed according to provider/network design.

Static vs dynamic

• static addressing is intentionally assigned and remains fixed until changed;
• dynamic addressing is commonly supplied through DHCP.

The choice depends on the endpoint/service role.

APIPA

A Windows-style 169.254.x.x address can indicate automatic link-local addressing when expected DHCP configuration was not obtained. It is a useful clue, not proof the NIC is dead.

IPv6

IPv6 uses 128-bit addresses and hexadecimal notation. At A+ depth, recognize that IPv6 is not "a longer IPv4 address" and does not use the same private-address model. The technician should identify whether the endpoint/network expects IPv4, IPv6, or both.

Default gateway

The gateway is the next-hop router used for destinations outside the local IP network. A valid local address with a wrong/missing gateway can allow local communication while blocking remote paths.

Guided case

Endpoint A:

IPv4 192.168.40.25/24
Gateway 192.168.40.1

Endpoint B:

IPv4 169.254.33.18/16
Gateway none

If the design expects DHCP on 192.168.40.0/24, Endpoint B has strong evidence of a configuration-acquisition problem.

Student action

For six supplied endpoint states classify:

• IPv4 private/public;
• IPv6 present/absent;
• static/dynamic expectation;
• subnet/prefix plausibility;
• gateway plausibility;
• APIPA clue;
• next evidence step.

Evidence checkpoint

You should be able to explain why address, subnet/prefix, gateway, addressing mode, and IP version are separate configuration decisions.