LPIC-2 205.1 — Basic Networking Configuration

Exam topic 205.1 — Basic Networking Configuration (weight: 3). Covers configuring wired and wireless network interfaces, routing tables, ARP, and the loopback interface.

Utilities and Files

ifconfig — Interface Configuration

ifconfig configures an interface and makes it available to the kernel’s network stack. It can assign an IP address, subnet mask, broadcast address, and activate or deactivate the interface.

Syntax

# Assign IP and bring interface up
ifconfig <interface> <ip-address>

# Assign IP with explicit mask
ifconfig eth0 172.16.1.2 netmask 255.255.255.0

# Bring interface up or down
ifconfig eth0 up
ifconfig eth0 down

# Show all interfaces (including inactive)
ifconfig -a

# Show a specific interface
ifconfig lo

Example output for eth0

eth0  Link encap:10Mps Ethernet  HWaddr 00:00:C0:90:B3:42
      inet addr:172.16.1.2  Bcast:172.16.1.255  Mask:255.255.255.0
      UP BROADCAST RUNNING  MTU:1500  Metric:1
      RX packets:0 errors:0 dropped:0 overrun:0
      TX packets:0 errors:0 dropped:0 overrun:0

Important: If netmask is not specified explicitly, ifconfig infers the mask from the address class. For 172.16.1.2 (class B) it would set 255.255.0.0 instead of the intended 255.255.255.0. Always specify the mask explicitly when working with subnets.

Tip: Short commands ifup <interface> and ifdown <interface> quickly bring up or down an already-configured interface.

route — Routing Table

route adds and removes routes in the kernel routing table.

Syntax

# Add/delete a route
route {add|del} [-net|-host] target [gw gateway] [netmask mask] [dev interface]

# Add a route to a network via a gateway
route add -net 172.16.0.0 netmask 255.255.255.0 gw romeo

# Add the default route
route add default gw 192.168.1.1

# Show the routing table
route

Example output

Kernel IP routing table
Destination  Gateway         Genmask        Flags Metric Ref Use Iface
default      192.168.1.254   0.0.0.0        UG    0      0   0   eth0
192.168.1.0  *               255.255.255.0  U     1      0   0   eth0

Warning: You can configure many gateways, but only one default gateway is active. default is shorthand for 0.0.0.0, which matches any destination when no more specific route exists.

Loopback Interface

The loopback (lo) is a virtual interface implemented entirely in the kernel’s network stack. It is not connected to any physical network. Packets sent to 127.0.0.1 (IPv4) or ::1 (IPv6) are simply returned up the stack as if they came from another device.

# Bring up loopback
ifconfig lo 127.0.0.1

# Check
ifconfig lo

Entry in /etc/hosts:

127.0.0.1  localhost

Important: Loopback must be configured even when the machine is not connected to any network. Without it, RPC applications (NFS, NIS) will fail to start — they register with portmapper via the loopback address.

Routing via a Gateway

If a network has multiple Ethernet segments connected by a gateway, add a route to the remote network. For example, if host romeo connects two networks:

# Add a route to 172.16.0.0 via gateway romeo
route add -net 172.16.0.0 netmask 255.255.255.0 gw romeo

# Make romeo the default gateway
route add default gw romeo

Warning: Make sure hosts in the remote network know the return route to your network. Otherwise packets will be sent but replies will never arrive.

ip — Modern Replacement for ifconfig and route

/sbin/ip is the modern tool for managing addresses, routes, tunnels, and policy routing. Officially recommended over ifconfig and route, though both legacy tools still work.

Working with addresses

# Show all addresses (like ifconfig -a)
ip addr show

# Assign an address to an interface
ip addr add 192.168.123.15/24 broadcast 192.168.123.255 dev eth0

# Remove an address
ip addr del 192.168.123.15/24 dev eth0

Working with routes

# Show routing table (like route)
ip route show

# Add a route to a network
ip route add 192.168.1.0/24 dev eth0

# Add a route via a gateway
ip route add 192.168.123.254/24 dev eth0

# Add default route
ip route add default via 192.168.1.1

Example output of ip addr show

1: lo: <LOOPBACK,UP,LOWER_UP> mtu 16436 qdisc noqueue state UNKNOWN
    link/loopback 00:00:00:00:00:00 brd 00:00:00:00:00:00
    inet 127.0.0.1/8 scope host lo
    inet6 ::1/128 scope host

2: eth0: <BROADCAST,MULTICAST,UP,LOWER_UP> mtu 1500 qdisc pfifo_fast state UP
    link/ether 00:90:f5:b6:91:d1 brd ff:ff:ff:ff:ff:ff
    inet 192.168.123.181/24 brd 192.168.123.255 scope global eth0
    inet6 fe80::290:f5ff:feb6:91d1/64 scope link

ARP — Address Resolution Protocol

In the OSI model, networking operates at two layers: Layer 2 (Ethernet, MAC addresses) and Layer 3 (IP addresses). To send an IP packet on a local network, the kernel needs the recipient’s MAC address. That is ARP’s purpose.

When the kernel wants to communicate with a host on the same subnet, it broadcasts an ARP request: “Who has IP address X? Send me your MAC.” The host with that IP replies with its MAC. The result is cached in the ARP cache.

# Show ARP cache
arp

# Example output:
Address       HWtype  HWaddress           Flags Mask  Iface
10.9.8.126    ether   00:19:bb:2e:df:73   C           wlan0

Note: There is also RARP (Reverse ARP). ARP: know the IP, get the MAC. RARP: know the MAC, get the IP.

Warning: Manually adding entries to the ARP cache is rarely needed — it updates automatically. To delete a stale entry: arp -d <ip>.

Wireless: iw, iwconfig, iwlist

iw (modern, nl80211)

iw works through the nl80211 standard (netlink). If iw does not see your device, the driver does not support nl80211 — use iwconfig instead.

# Show wireless interfaces
iw dev

# Connection status
iw dev wlan0 link

# Scan for networks
iw dev wlan0 scan

# Connect to an access point
iw dev wlan0 connect "MyNetwork"

# Connect specifying a channel
iw dev wlan0 connect "MyNetwork" 2432

# Connect with a key
iw dev wlan0 connect "MyNetwork" 0:"mypassword"

# Switch to Ad-Hoc mode
iw dev wlan0 set type ibss

# Enable power saving
iw dev wlan0 set power_save on

iwconfig (legacy, WEXT)

iwconfig works similarly to ifconfig but only for wireless interfaces. The wireless_tools package is formally deprecated but still widely supported.

# Show wireless interface parameters
iwconfig wlan0

# Connect to a network by ESSID
iwconfig wlan0 essid "MyNetwork"

# Set encryption key (ASCII)
iwconfig wlan0 key s:mypassword

# Disable ESSID check
iwconfig wlan0 essid any

# Switch operating mode
iwconfig wlan0 mode Ad-Hoc
iwconfig wlan0 mode Managed

Note: Mode values: Ad-Hoc — network without an access point; Managed — client with roaming; Master — access point mode; Monitor — passive monitoring without association.

iwlist (scanning)

# Scan available networks
iwlist wlan0 scan

# Show supported encryption keys
iwlist wlan0 keys

Warning: Scanning (scan) requires root. When run as a regular user, iwlist returns the results of the last scan if available.

Exam Cheat Sheet

Key Commands — old vs new

Files and Paths

Common Mistakes

• Forgetting netmask in ifconfig — the mask will be inferred from the address class, likely wrong.
• Configuring two default gateways — only one works; the second is ignored.
• Using iw with a WEXT-driver card — switch to iwconfig if iw doesn’t see the device.
• Running iwlist scan as a regular user — root is required for scanning.

Practice Questions

Q1. How do you assign IP 192.168.10.5/24 to eth0 using ifconfig?

Answer: ifconfig eth0 192.168.10.5 netmask 255.255.255.0

Q2. How do you add default gateway 10.0.0.1 using route?

Answer: route add default gw 10.0.0.1

Q3. What is the difference between iw and iwconfig?

Answer: iw uses the nl80211 (netlink) standard; iwconfig uses the legacy WEXT standard from the wireless_tools package. If the card driver does not support nl80211, iw will not see the device — use iwconfig instead.

Q4. Which command shows the ARP cache?

Answer: arp or ip neigh show

Q5. What does the default route in the routing table mean?

Answer: default is shorthand for 0.0.0.0 — it matches any destination when no more specific route exists. Only one default gateway can be active at a time.

Q6. Which IPv6 address identifies the loopback interface?

Answer: ::1

Q7. Why must the loopback interface be configured even when the machine is not connected to any network?

Answer: RPC applications (NFS, NIS) register with portmapper via the loopback address 127.0.0.1 at startup. Without loopback, these services will not start.

Q8. How do you show all addresses assigned to network interfaces using ip?

Answer: ip addr show