How Your Machine Is Connected

Addresses

Every device on a network has an IP address. An IPv4 address is four numbers from 0 to 255 separated by dots, like 192.168.1.23, 32 bits in all; an IPv6 address is eight groups of four hex digits separated by colons, 128 bits ¹. Most home networks use both; IPv4 is the one you’ll type.

An address comes with a netmask, usually written as a slash and a number. In 192.168.1.23/24, the first 24 bits (192.168.1) name the network and the last 8 name the device on it, so everything from 192.168.1.0 to 192.168.1.255 is the same local network ¹.

Home networks use private ranges, set aside for networks that don’t face the internet directly: 10.0.0.0/8, 172.16.0.0/12, and 192.168.0.0/16 ². Your router has one public address on the internet side and shares it among all your devices.

Interfaces

Each network connection is an interface. Ubuntu names wired ones predictably, from where the hardware sits, like eno1 or enp0s25; Wi-Fi interfaces start with wl, like wlp2s0; and lo, the loopback, is the machine talking to itself at 127.0.0.1 ³. ip addr shows them all, from Ubuntu’s documentation ^{3 4}:

$ ip addr
1: lo: <LOOPBACK,UP,LOWER_UP> mtu 65536 qdisc noqueue state UNKNOWN group default qlen 1000
    link/loopback 00:00:00:00:00:00 brd 00:00:00:00:00:00
    inet 127.0.0.1/8 scope host lo
       valid_lft forever preferred_lft forever
    inet6 ::1/128 scope host
       valid_lft forever preferred_lft forever
2: enp0s25: <BROADCAST,MULTICAST,UP,LOWER_UP> mtu 1500 qdisc noqueue state UP group default qlen 1000
    link/ether 00:16:3e:e2:52:42 brd ff:ff:ff:ff:ff:ff link-netnsid 0
    inet 10.102.66.200/24 brd 10.102.66.255 scope global dynamic eth0
       valid_lft 3257sec preferred_lft 3257sec
    inet6 fe80::216:3eff:fee2:5242/64 scope link
       valid_lft forever preferred_lft forever

For each interface: its state (UP), its hardware MAC address (link/ether), its IPv4 address with netmask (inet 10.102.66.200/24), and IPv6 addresses (inet6). dynamic means it was handed out by DHCP, and valid_lft is how long until it must be renewed. ip -br addr gives a one-line-per-interface summary.

Routes and the gateway

Anything for the local network goes straight there. Everything else goes to the gateway, normally your router, which passes it on toward the internet ¹. ip route shows the routing table ⁵; the line that matters is the default route. On a typical home network:

default via 192.168.1.1 dev wlp2s0 proto dhcp metric 600
192.168.1.0/24 dev wlp2s0 proto kernel scope link src 192.168.1.23 metric 600

The first line sends everything not otherwise matched to 192.168.1.1 through the Wi-Fi interface; the second says the 192.168.1.0/24 network is reachable directly. If the default route is missing, local things work and the internet doesn’t.

The router usually does three jobs: hands out addresses by DHCP, acts as the gateway, and answers or passes on DNS questions ¹.

Names

People use names; packets use addresses. DNS turns one into the other ¹. On Ubuntu, a local service, systemd-resolved, handles lookups for every program, with a stub listener at 127.0.0.53 ⁶. resolvectl asks it questions ⁷:

me@garden-laptop:~$ resolvectl status
me@garden-laptop:~$ resolvectl query example.com

status shows which DNS servers each interface is using, usually your router; query looks a name up and shows the address ⁷.

Two more sources of names:

• /etc/hosts maps names to addresses by hand, one line per host ⁸. Names listed there work even when DNS doesn’t know them, handy for a machine on your own network.
• .local names. Avahi lets machines on the same network find each other by name, as hostname.local, with no DNS server involved ⁹. If it’s running on both, the server named garden-server is reachable as garden-server.local.

NetworkManager

On Ubuntu Desktop, NetworkManager runs the connections, behind the network menu at the top right, and nmcli is its command-line side ¹⁰:

me@garden-laptop:~$ nmcli device status
me@garden-laptop:~$ nmcli device wifi list
me@garden-laptop:~$ nmcli --ask device wifi connect "Garden Wi-Fi"

device status lists interfaces and their connections; wifi list shows networks in range; wifi connect joins one; --ask makes nmcli prompt for the password, so it doesn’t land in your shell history ¹⁰. Ubuntu Server uses Netplan files in /etc/netplan/ instead ¹.

Testing a connection

me@garden-laptop:~$ ping -c 3 192.168.1.1
me@garden-laptop:~$ ss -tlnp

ping sends small echo requests and reports the replies and round-trip times; -c 3 stops after three ¹¹. Some devices ignore pings, so silence isn’t always failure.

ss lists network sockets ¹². With -t (TCP), -l (listening only), -n (numbers, not names), and -p (which program, needing sudo for others’ programs), it shows what’s waiting for connections on this machine: port 22 for an SSH server, 631 for printing, and so on ¹².

A fixed address for a server

DHCP addresses can change. For a machine others connect to, like the home server this course ends with, give it a fixed address. The simplest way is in your router’s settings: a “DHCP reservation” (sometimes “address reservation” or “static lease”) ties an address to the machine’s MAC address from ip addr. Every router’s admin page is different; the name of the setting is what to look for. With .local names working, you may not even need it.

When something doesn’t work

Climb the ladder from the bottom, and stop at the first rung that fails:

1. Link. Is the cable plugged in, or Wi-Fi joined? nmcli device status says connected or not.
2. Address. Does ip addr show an inet address on that interface? If there’s none, or it’s in 169.254.0.0/16, the range a device gives itself when it can’t get one by DHCP, DHCP didn’t work ².
3. Gateway. Does ping -c 3 to the router’s address (from ip route) get replies?
4. Names. Does resolvectl query example.com return an address? If pinging an address works but names fail, it’s DNS.
5. Service. On the machine you’re connecting to, is the program listening? ss -tlnp. Is a firewall in the way (module 6)?

Check the journal too: journalctl -b -u NetworkManager.

Your turn

So

Each interface (lo, enp..., wlp...) has an address and netmask, usually from DHCP; home networks use private ranges like 192.168.0.0/16. ip addr shows addresses, ip route the default route to the gateway (your router), resolvectl the DNS in use through systemd-resolved, nmcli NetworkManager’s connections, ping whether something answers, and ss -tlnp what’s listening. /etc/hosts and .local names add names without DNS. Give a server a fixed address with a reservation in the router. When it breaks, check link, address, gateway, names, then service.