Get network configuration

A usable host configuration normally includes four pieces: an address, a prefix, routes, and DNS server information. The exact source can be static configuration or an automatic protocol, but a host needs all four to be genuinely online.

How IPv4 gets configured: DHCP

DHCP commonly supplies IPv4 configuration through a discover, offer, request, and acknowledgment exchange. Your machine broadcasts “anyone have an address for me?”, a DHCP server offers one, your machine formally requests it, and the server acknowledges. A lease gives the configuration a limited lifetime, after which the host must renew it.

On macOS you can read the actual lease your machine received:

ipconfig getpacket en0

yiaddr = 192.168.1.20
subnet_mask (ip): 255.255.255.0
router (ip_mult): {192.168.1.1}
domain_name_server (ip_mult): {192.168.1.1}
lease_time (uint32): 0x15180

There are the four pieces in one packet: your address (yiaddr), the prefix (as a subnet mask), the router, and the DNS server. The lease time 0x15180 is 86400 seconds — one day. On Linux, journalctl -u NetworkManager | grep -i dhcp or your distribution’s equivalent shows the same negotiation in logs.

How IPv6 gets configured: Router Advertisements

IPv6 hosts can learn prefixes and default routers from Router Advertisements. Stateless Address Autoconfiguration (SLAAC) lets a host form its own address for an advertised prefix — no server keeps a list of who has what.

Check the results:

ip -6 address show eth0
# inet6 2a01:e0a:4f2:1:8c3a:11ff:fe2b:940c/64 scope global dynamic
ip -6 route | grep default
# default via fe80::9e53:22ff:feaa:41e dev eth0 proto ra

The proto ra tag on the route means it was learned from a Router Advertisement, not typed in by anyone.

Verify the pieces agree

Configuration succeeded only when the pieces agree. An address without a suitable route may reach the local link but nowhere else. A working IP path without DNS can reach addresses but not resolve names.

A quick three-part check:

ip address        # do I have an address and prefix?
ip route          # do I have a default route?
dig example.com   # can I resolve names?

When someone says “the network is down”, run these three. Address present but no default route: local misconfiguration or DHCP handed out something incomplete. Address and route fine but dig times out: it’s DNS, not the network. Each failure pattern names its own culprit.