Addressing#
Three screens put an address on an interface: the interface panel, Adding an interface, and the add-uplink wizard on Uplinks. A fourth, Networks, gives a VLAN its router address along with its DHCP scope. This page is what an address may be, and what each choice then implies.
An address#
An address is written with its prefix length: 192.0.2.1/24 for IPv4,
2001:db8:1::1/64 for IPv6. 192.0.2.1 on its own is refused under the field, before
anything is staged. A host address on a dummy interface is a /32, or /128 for IPv6.
An interface can hold several addresses, and the panel's Addresses list is all of the static ones: add one in the box, remove one with its ×. Moving an interface to a new address is removing the old one and adding the new one in the same save.
Address from DHCP and IPv6 address from DHCPv6 are switches beside the list: the router runs a client on the interface and takes what it is given.
Which addresses the pages show#
Every interface has two lists, and they mean different things.
- Configured — what the configuration declares: the panel's Addresses list.
- Live — what the router holds. An address from a DHCP lease is here and not in the configured list, because the configuration only says to use DHCP.
The Interfaces table shows the live list without link-local addresses; the panel's Status shows it whole. An address configured but not present, or present but not configured, is drift, and the disagreement is the signal.
What taking an address from DHCP does#
A DHCP client does more than fetch an address: it also installs a default route through the gateway the lease names, which turns the interface into an uplink as far as the rest of the product is concerned. That route's administrative distance is 210 unless the interface says otherwise, which is why a DHCP uplink loses to a static default route at distance 1. A PPPoE session's route has distance 1 by default. Both are set on Uplinks, which also lists every interface carrying a default route.
IPv6#
An IPv6 address is added the same way and applied the same way. Its addresses, router advertisements and delegated prefixes are on IPv6; handing addresses out with DHCPv6 is on DHCP and IPv6.
- The IPv6 forwarding table is on Static routes, beside the IPv4 one, and on IPv6.
- The uplink model has no IPv6 concept. It looks for default routes named
0.0.0.0/0and nothing else. - A network's access policy on Networks covers IPv4 only.
MTU#
The MTU field on the interface panel starts as the configuration has it; empty is the default, 1500. It takes 68 to 16000 bytes.
| Value | When |
|---|---|
| 1500 | plain Ethernet — the default, and right almost everywhere |
| 1492 | an interface behind PPPoE, which spends 8 bytes on its own header |
| up to 9000 | jumbo frames, on a path where every device agrees |
Nothing adjusts the path MTU for you and nothing warns you. An MTU larger than the smallest link on the path produces the classic failure: pings work, DNS works, and anything that fills a packet — a TLS handshake, a file copy — hangs. If you raise the MTU on one interface, raise it on the switch and on the far end, and test with a full-size packet before you rely on it. A VLAN's MTU cannot be larger than its parent's.
Lowering an MTU takes effect immediately and breaks nothing. Raising it is the one to be careful with.
What breaks if you get it wrong#
- Two interfaces in the same subnet. Return traffic picks an interface by the routing table and it may not be the one you meant. Networks refuses a subnet that overlaps another interface's; the interface panel does not.
- DHCP on a LAN interface. The router becomes a DHCP client on a segment where it was meant to be the server, and installs a default route through it at distance 210, which Uplinks then lists as an uplink.
- Changing the address you are connected through. The session ends when the change is applied. Apply with auto-revert: if the new address is wrong you get the old configuration back rather than a router that answers on nothing.
- An address the firewall does not expect. Rules that match on a source prefix keep matching the prefix, not the interface. Moving a segment to a new prefix means editing the rules too — which Networks does for its own.
See also#
- The interface panel — where addresses are added and removed.
- Adding an interface — the address field on a new interface.
- Uplinks (multi-WAN) — what DHCP, PPPoE and a static gateway each do to the default route.
- Static routes — administrative distance, and which route wins.
- DHCP — the other end of the same protocol: handing addresses out.