IP addressing on interfaces — static, DHCP, and address-less

An Ethernet interface has three addressing modes: static operator-configured address, DHCP client (the interface dynamically gets an address from a server), and address-less (the interface is up but has no L3 address, used for bridging or L2-only purposes). This lesson covers all three and the failure modes that surface when an addressing mistake locks the operator out.

Static IPv4 addressing

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vyos@vyos# set interfaces ethernet eth0 address '192.0.2.1/24'
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vyos@vyos# commit
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vyos@vyos# save

The interface is configured with 192.0.2.1/24. The kernel notifies the connected route is installed in the routing table.

vyos@vyos:~$ show interface ethernet eth0
eth0: <BROADCAST,MULTICAST,UP,LOWER_UP> mtu 1500 qdisc fq_codel state UP group default qlen 1000
    link/ether 52:54:00:12:34:56 brd ff:ff:ff:ff:ff:ff
    inet 192.0.2.1/24 brd 192.0.2.255 scope global eth0
       valid_lft forever preferred_lft forever

The inet 192.0.2.1/24 line confirms the address is installed.

DHCP client

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vyos@vyos# set interfaces ethernet eth0 address 'dhcp'
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vyos@vyos# commit
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vyos@vyos# save

The interface runs a DHCP client. The client sends DHCPDISCOVER, receives DHCPOFFER, sends DHCPREQUEST, receives DHCPACK. The address, gateway, and DNS are populated.

vyos@vyos:~$ show dhcp client leases
Interface  eth0
IP address  203.0.113.42/24
Gateway     203.0.113.1
DNS server  1.1.1.1, 9.9.9.9
Lease obtained   2026-08-15 14:23:01
Lease expires    2026-08-15 22:23:01

The show dhcp client leases output shows the lease details.

Static IPv6 addressing

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vyos@vyos# set interfaces ethernet eth0 address '2001:db8::1/64'
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vyos@vyos# commit
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vyos@vyos# save

IPv6 addresses follow the same syntax as IPv4. The /64 prefix is the standard for a single subnet.

DHCPv6 client

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vyos@vyos# set interfaces ethernet eth0 address 'dhcpv6'
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vyos@vyos# commit
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vyos@vyos# save

The interface runs a DHCPv6 client. DHCPv6 is stateless for address assignment (the prefix comes from Router Advertisements) but stateful for DNS and other options.

Multiple addresses on a single interface

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vyos@vyos# set interfaces ethernet eth0 address '192.0.2.1/24'
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vyos@vyos# set interfaces ethernet eth0 address '192.0.2.10/24'
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vyos@vyos# commit
[edit]
vyos@vyos# save

Multiple addresses on a single interface are valid. The kernel adds both; the routing table shows two connected routes for the same subnet (the more specific one wins for traffic from those addresses, but both routes cover the subnet).

Address-less interfaces

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vyos@vyos# set interfaces ethernet eth0
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vyos@vyos# commit
[edit]
vyos@vyos# save

An interface with no address directive is address-less. The interface is up at L2 but has no L3 address. Use cases:

• Bridging: the interface is part of a bridge; the bridge has the address.
• Trunk port: the interface is a VLAN trunk; the VLAN sub-interfaces have the addresses.
• Transit-only: the interface forwards traffic but is not the source or destination.

How the result is validated

show interface ethernet
ip addr show
show dhcp client leases
show dhcpv6 client leases
show ip route 192.0.2.0/24

The first shows the VyOS view; the second shows the kernel view; the third/fourth show DHCP state; the fifth shows the connected route.

How it fails

The production failure modes the engineer must recognise:

• Address on the wrong interface. A box configured with the LAN address on eth0 and the WAN address on eth1 is unreachable from the WAN.
• Duplicate address. Two interfaces configured with the same address. The kernel accepts the second but traffic is unpredictable.
• Wrong prefix length. A /24 configured as /32 produces a host route, not a subnet route. Traffic to the subnet does not go through the box.
• DHCP client never receives a lease. The client retries indefinitely; the interface stays without an address. The operator must verify the upstream DHCP server is reachable.
• Address-less interface used for routing. An interface configured as a routed interface but without an address has no connected route and no routing entry. Routing breaks.

Rollback

The recovery from a bad address configuration:

• Wrong static: delete interfaces ethernet eth0 address '192.0.2.1/24'; set interfaces ethernet eth0 address '192.0.2.5/24'; commit; save.
• Wrong DHCP/static mix: delete interfaces ethernet eth0 address 'dhcp'; set interfaces ethernet eth0 address '192.0.2.1/24'; commit; save.
• Duplicate address: delete the duplicate, commit; save.

Production discipline

Cross-course references

The Linux course’s XIX-Linux-NetFoundations covers the kernel side of IP addressing. The OPNsense course’s VII-OPNsense-Interfaces covers the equivalent configuration on the firewall side. The Observability course’s LX-Observability-NetworkObs covers how to alert on interface state changes.

Quiz