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Interfaces/Bridge, IPv4/IPv6 Addressing

Network interfaces are the points of connection between a host and a network. Linux provides a rich set of tools for configuring, bonding, bridging, and virtualising network interfaces.

Network Interfaces

Every network interface in Linux is represented by a kernel interface object. Physical interfaces correspond to hardware NICs; virtual interfaces are created in software.

List interfaces with ip:

ip link show
ip addr show

Common Interface Types

  • eth0, enp0s3: Physical Ethernet interfaces.
  • wlan0, wlp2s0: Wireless interfaces.
  • lo: Loopback interface (127.0.0.1).
  • docker0, br0: Virtual bridges.
  • veth0: Virtual Ethernet pairs (used by containers).
  • tun0, tap0: Tunnels (VPN).
  • bond0: Bonded interfaces (link aggregation).
  • vlan10: VLAN-tagged interfaces.

Configuring Interfaces

ip addr add 192.168.1.10/24 dev eth0
ip link set eth0 up
ip route add default via 192.168.1.1

Persistent configuration is typically managed by NetworkManager, systemd-networkd, or netplan.

Bridges

A bridge connects multiple network segments at the data link layer (Layer 2). It makes multiple interfaces appear as a single interface, forwarding frames between them.

Create a bridge:

ip link add name br0 type bridge
ip link set eth0 master br0
ip link set eth1 master br0
ip addr add 192.168.1.10/24 dev br0
ip link set br0 up

Bridges are commonly used with virtual machines and containers.

Bonding combines multiple physical interfaces into a single logical interface for redundancy or increased bandwidth.

Common modes:

  • active-backup: One interface active, others standby.
  • balance-rr: Round-robin distribution.
  • 802.3ad (LACP): Link Aggregation Control Protocol (requires switch support).
ip link add name bond0 type bond mode active-backup
ip link set eth0 master bond0
ip link set eth1 master bond0

IPv4 Addressing

IPv4 addresses are 32-bit numbers, typically written in dotted-decimal notation (e.g., 192.168.1.10).

Address Classes

  • Class A: 1.0.0.0 to 126.255.255.255 (large networks).
  • Class B: 128.0.0.0 to 191.255.255.255 (medium networks).
  • Class C: 192.0.0.0 to 223.255.255.255 (small networks).

Private Address Ranges

Private addresses are not routed on the public internet:

  • 10.0.0.0/8
  • 172.16.0.0/12
  • 192.168.0.0/16

Subnetting

A subnet mask divides an IP address into network and host portions. CIDR notation combines address and mask: 192.168.1.0/24.

IPv6 Addressing

IPv6 addresses are 128-bit numbers, written in hexadecimal with colons (e.g., 2001:db8::1).

Address Types

  • Global Unicast: Publicly routable addresses.
  • Link-Local: fe80::/10, automatically configured on every interface.
  • Unique Local: fc00::/7, similar to IPv4 private addresses.
  • Loopback: ::1, equivalent to 127.0.0.1.
  • Multicast: ff00::/8, for one-to-many communication.

IPv6 Autoconfiguration

IPv6 supports Stateless Address Autoconfiguration (SLAAC), where hosts generate their own addresses using router advertisements:

sysctl net.ipv6.conf.eth0.accept_ra=1