4.4 Your Computer on the Network

Your Computer on the Network (Objective 4.4)

Linux Essentials expects a clear mental model of networking—not Cisco-level routing design. You should know how a host gets an address, how it finds a default gateway, how names become addresses, and which commands show that information.

Networks, routers, and the Internet

A network is a group of devices that can reach each other under shared addressing and local forwarding rules (your home LAN or office VLAN is a network). A router (or gateway) forwards packets between networks—for example, from your LAN to your ISP. The Internet is the global interconnection of many networks using standard protocols (especially IP). Your laptop is one host on a local network; when you browse a website, packets typically leave via a default route toward a router that can reach the wider Internet.

Addressing: IPv4 and IPv6

Interfaces may have IPv4, IPv6, or both (dual stack). Essentials does not require calculating subnet masks by hand, but you should recognize which notation is which and that both can appear in ip/ifconfig output.

Showing addresses: ip and ifconfig

On current distributions, the ip command from iproute2 is preferred. ifconfig may be missing unless a legacy package is installed, but exam questions may still mention it as the older equivalent for “show my IP.” When reading output, identify the interface name (eth0, ens33, wlan0, …) and the address lines.

Routes: ip route and route

The routing table answers: “For this destination, which interface and next hop?” The default route (often shown as default or 0.0.0.0/0) points at your gateway/router for destinations not on the local network. Without a working default route (and a reachable gateway), local LAN traffic might work while Internet sites fail.

Connections and sockets: netstat and ss

| Command | Role | |---|---|---| | ss | Modern socket statistics (listening ports, connections) | | netstat | Legacy counterpart; still referenced widely |

Essentials-level use is awareness: these tools list network sockets—for example, which ports are listening or which remote hosts you are connected to. You do not need every flag; know that ss replaces netstat on many systems.

Name resolution: /etc/resolv.conf and /etc/hosts

Humans use names (example.com); IP stacks need addresses. Two classic files matter:

If /etc/resolv.conf points at unreachable resolvers, browsing by name fails even when ping to a raw IP works. /etc/hosts is useful for small local overrides (lab VMs, blocked names, short aliases). On systems using systemd-resolved or NetworkManager, resolv.conf may be managed automatically—you still need to know what the file is for.

Checking reachability and DNS: ping and host

ping answers “can I reach this IP/host at the IP layer (if ICMP is allowed)?” Firewalls may block ping even when a service is up, so a failed ping is a clue—not absolute proof the host is down. host answers “what address does DNS return for this name?” Related tools (dig, nslookup) exist; Essentials highlights host in the objective set.

A simple connectivity checklist

1. ip addr show — Do I have an address on the right interface?
2. ip route show — Is there a default gateway?
3. ping the gateway, then an Internet IP (if allowed).
4. Check /etc/resolv.conf, then host example.com (or similar).
5. Remember /etc/hosts if a single name behaves differently from DNS.

Common exam traps

• Confusing /etc/hosts (local static map) with /etc/resolv.conf (which DNS servers to ask).
• Assuming ifconfig is always installed; know ip addr show as the modern equivalent.
• Thinking a failed ping always means “host down”—ICMP may be filtered.
• Mixing up routing (ip route show) with address assignment (ip addr show).

Private lab addresses (for example ranges reserved for local use) still appear in the same command output as public addresses; Essentials cares that you can read the tools, not that you memorize every RFC1918 block.

Putting name resolution and reachability together

Separate layer problems when you practice. If ping 203.0.113.10 works but ping www.example.com fails, suspect DNS: read /etc/resolv.conf, then try host www.example.com. If even the numeric IP fails, check address and route with ip addr show and ip route show, then ping the gateway. If one short name resolves oddly while others are fine, inspect /etc/hosts for a stale static line. Legacy names matter too: an older tutorial may say ifconfig, route, or netstat—map those to ip addr, ip route, and ss so you are not stuck when the modern binary is what your distro ships.

Exam focus

Match tasks to tools: display address → ip addr show/ifconfig; show routes → ip route show/route; resolver config → /etc/resolv.conf; local static names → /etc/hosts; reachability → ping; DNS lookup → host. Know IPv4 vs IPv6 notation and that routers interconnect networks to form paths toward the Internet.