Linux Networking: Viewing and Configuring Your Network

Table of Contents

linux-networking - This article is part of a series.

Part 1: This Article

Part 2: Linux Firewalls: Controlling Traffic with iptables and nftables

Part 3: Linux Network Troubleshooting: A Systematic Approach

Your Network at a Glance
#

Before troubleshooting or configuring anything, you need to know what your current network setup looks like. Linux provides the ip command as the primary tool for this — it replaces the older ifconfig and route commands.

Viewing Network Interfaces
#

An interface is a connection point to a network — physical (Ethernet, Wi-Fi) or virtual (loopback, VPN tunnels).

# List all interfaces and their state
ip link show

Output:

1: lo: <LOOPBACK,UP,LOWER_UP> mtu 65536 state UNKNOWN
    link/loopback 00:00:00:00:00:00 brd 00:00:00:00:00:00
2: eth0: <BROADCAST,MULTICAST,UP,LOWER_UP> mtu 1500 state UP
    link/ether 52:54:00:ab:cd:ef brd ff:ff:ff:ff:ff:ff
3: wlan0: <BROADCAST,MULTICAST> mtu 1500 state DOWN
    link/ether a0:b1:c2:d3:e4:f5 brd ff:ff:ff:ff:ff:ff

Field	Meaning
`lo`	Loopback (localhost, always present)
`eth0`	First Ethernet interface
`wlan0`	Wireless interface
`UP`	Interface is enabled
`DOWN`	Interface is disabled
`state UP`	Link is connected
`mtu 1500`	Maximum packet size (bytes)
`link/ether`	MAC address

Finding Your IP Address
#

# Show all addresses on all interfaces
ip addr show

# Short form
ip a

# Show addresses for a specific interface
ip addr show eth0

Output:

2: eth0: <BROADCAST,MULTICAST,UP,LOWER_UP> mtu 1500
    link/ether 52:54:00:ab:cd:ef brd ff:ff:ff:ff:ff:ff
    inet 192.168.1.50/24 brd 192.168.1.255 scope global dynamic eth0
    inet6 fe80::5054:ff:feab:cdef/64 scope link

Field	Meaning
`inet 192.168.1.50/24`	IPv4 address and subnet mask (CIDR notation)
`/24`	Subnet mask = 255.255.255.0 (first 24 bits are the network)
`brd 192.168.1.255`	Broadcast address
`dynamic`	Assigned by DHCP
`inet6 fe80::...`	IPv6 link-local address
`scope global`	Routable address
`scope link`	Only valid on this network segment

CIDR to subnet mask reference
#

CIDR	Subnet Mask	Usable Hosts
`/8`	255.0.0.0	16,777,214
`/16`	255.255.0.0	65,534
`/24`	255.255.255.0	254
`/25`	255.255.255.128	126
`/30`	255.255.255.252	2

Quick one-liner for just the IP
#

ip -4 addr show eth0 | grep -oP '(?<=inet\s)\d+(\.\d+){3}'

Or more readably:

hostname -I

Finding Your Default Gateway
#

The gateway is the router that forwards traffic to other networks (the internet):

ip route show

Output:

default via 192.168.1.1 dev eth0 proto dhcp metric 100
192.168.1.0/24 dev eth0 proto kernel scope link src 192.168.1.50

Line	Meaning
`default via 192.168.1.1`	Default gateway — all traffic not matching other routes goes here
`dev eth0`	Via this interface
`192.168.1.0/24 dev eth0`	The local subnet is directly connected

Just the gateway IP
#

ip route show default | awk '{print $3}'

Finding Your DNS Servers
#

DNS servers translate domain names to IP addresses. Check your configuration:

# Modern systems using systemd-resolved
resolvectl status

# Or check the traditional file
cat /etc/resolv.conf

/etc/resolv.conf output:

nameserver 192.168.1.1
nameserver 8.8.8.8

On systems with systemd-resolved, the actual resolvers may differ from what’s in /etc/resolv.conf. Use resolvectl status for the truth.

Testing Connectivity
#

`ping` — Can you reach a host?
#

# Ping by IP (tests Layer 3 — routing)
ping -c 4 8.8.8.8

# Ping by hostname (tests DNS + routing)
ping -c 4 google.com

# Ping your gateway (first thing to check when things break)
ping -c 4 192.168.1.1

The -c 4 flag sends 4 packets and stops. Without it, ping runs until you press Ctrl-c.

Troubleshooting with ping:

Result	Likely problem
Gateway responds, internet doesn’t	Routing or ISP issue
Nothing responds	Interface down, wrong IP, cable unplugged
IP works, hostname doesn’t	DNS problem

`traceroute` / `tracepath` — What path do packets take?
#

# Show every hop between you and the destination
traceroute google.com

# tracepath (doesn't require root)
tracepath google.com

Each line is a router between you and the target. Useful for finding where connectivity breaks.

`nc` (netcat) — Is a specific port reachable?
#

# Test if port 443 is open on a host
nc -zv google.com 443

# Test SSH on a server
nc -zv 192.168.1.100 22

-z scans without sending data, -v gives verbose output.

`ss` — What’s listening on your machine?
#

# Show all listening TCP ports
ss -tlnp

# Show all listening UDP ports
ss -ulnp

# Show all established connections
ss -tunap

Flag	Meaning
`-t`	TCP
`-u`	UDP
`-l`	Listening only
`-n`	Numeric (don’t resolve names)
`-p`	Show process using the port

Configuring a Static IP Address
#

Temporary (until reboot)
#

# Add an IP address to an interface
sudo ip addr add 192.168.1.100/24 dev eth0

# Remove an IP address
sudo ip addr del 192.168.1.100/24 dev eth0

Set the default gateway
#

# Add a default route
sudo ip route add default via 192.168.1.1 dev eth0

# Remove the default route
sudo ip route del default

Persistent with NetworkManager (nmcli)
#

Most desktop and server distributions use NetworkManager. Use nmcli for persistent changes:

# Show connections
nmcli connection show

# Set a static IP
nmcli connection modify "Wired connection 1" \
    ipv4.method manual \
    ipv4.addresses 192.168.1.100/24 \
    ipv4.gateway 192.168.1.1 \
    ipv4.dns "8.8.8.8,1.1.1.1"

# Apply the changes
nmcli connection up "Wired connection 1"

Switch back to DHCP
#

nmcli connection modify "Wired connection 1" \
    ipv4.method auto

nmcli connection up "Wired connection 1"

Persistent with netplan (Ubuntu server)
#

Ubuntu servers often use netplan. Edit /etc/netplan/01-config.yaml:

network:
  version: 2
  ethernets:
    eth0:
      addresses:
        - 192.168.1.100/24
      routes:
        - to: default
          via: 192.168.1.1
      nameservers:
        addresses:
          - 8.8.8.8
          - 1.1.1.1

Apply:

sudo netplan apply

DHCP vs Static
#

	DHCP	Static
How it works	Router assigns IP automatically	You configure IP manually
Use when	Desktops, laptops, most workstations	Servers, printers, infrastructure
Pros	Zero config, no conflicts	Predictable, won’t change
Cons	IP may change over time	Must track assignments yourself

Most machines use DHCP by default. Switch to static when you need a predictable address — for SSH access, running a web server, or DNS records.

Bringing Interfaces Up and Down
#

# Disable an interface
sudo ip link set eth0 down

# Enable an interface
sudo ip link set eth0 up

# Check state
ip link show eth0

With NetworkManager:

nmcli device disconnect eth0
nmcli device connect eth0

Viewing Your MAC Address
#

ip link show eth0 | grep link/ether
# link/ether 52:54:00:ab:cd:ef

Change it temporarily (useful for testing):

sudo ip link set eth0 down
sudo ip link set eth0 address 02:01:02:03:04:05
sudo ip link set eth0 up

Quick Reference
#

Task	Command
Show all interfaces	`ip link show`
Show IP addresses	`ip addr show`
Show default gateway	`ip route show default`
Show DNS servers	`resolvectl status`
Ping a host	`ping -c 4 host`
Trace route to host	`tracepath host`
Check open ports	`ss -tlnp`
Test a port	`nc -zv host port`
Set static IP (temp)	`sudo ip addr add IP/MASK dev IFACE`
Set gateway (temp)	`sudo ip route add default via GW`
Set static IP (permanent)	`nmcli connection modify ...`
Bring interface up	`sudo ip link set IFACE up`

Best Practices
#

Use ip commands instead of ifconfig and route — they’re deprecated on modern systems
Always test with ping to your gateway first — if that fails, the problem is local
Use -c with ping to avoid running forever
Set static IPs on servers so they don’t change when DHCP leases expire
Keep DNS settings separate from your router when possible — use 1.1.1.1 or 8.8.8.8 as a fallback
Remember that ip addr add is temporary — use nmcli or netplan for changes that survive reboot
Check ss -tlnp when troubleshooting services — if nothing is listening, the service isn’t running

linux-networking - This article is part of a series.

Part 1: This Article

Part 2: Linux Firewalls: Controlling Traffic with iptables and nftables

Part 3: Linux Network Troubleshooting: A Systematic Approach

Part 4: SSH Tunnels and Port Forwarding

Your Network at a Glance#

Viewing Network Interfaces#

Finding Your IP Address#

CIDR to subnet mask reference#

Quick one-liner for just the IP#

Finding Your Default Gateway#

Just the gateway IP#

Finding Your DNS Servers#

Testing Connectivity#

ping — Can you reach a host?#

traceroute / tracepath — What path do packets take?#

nc (netcat) — Is a specific port reachable?#

ss — What’s listening on your machine?#

Configuring a Static IP Address#

Temporary (until reboot)#

Set the default gateway#

Persistent with NetworkManager (nmcli)#

Switch back to DHCP#

Persistent with netplan (Ubuntu server)#

DHCP vs Static#

Bringing Interfaces Up and Down#

Viewing Your MAC Address#

Quick Reference#

Best Practices#

Your Network at a Glance
#

Viewing Network Interfaces
#

Finding Your IP Address
#

CIDR to subnet mask reference
#

Quick one-liner for just the IP
#

Finding Your Default Gateway
#

Just the gateway IP
#

Finding Your DNS Servers
#

Testing Connectivity
#

`ping` — Can you reach a host?
#

`traceroute` / `tracepath` — What path do packets take?
#

`nc` (netcat) — Is a specific port reachable?
#

`ss` — What’s listening on your machine?
#

Configuring a Static IP Address
#

Temporary (until reboot)
#

Set the default gateway
#

Persistent with NetworkManager (nmcli)
#

Switch back to DHCP
#

Persistent with netplan (Ubuntu server)
#

DHCP vs Static
#

Bringing Interfaces Up and Down
#

Viewing Your MAC Address
#

Quick Reference
#

Best Practices
#