Updated: 28 September 2026 · Applies to: Self-managed dedicated servers on Linux and Windows

After you add extra IP addresses to a server, test each one before you rely on it. Three things must work: the address is configured on the server, it answers from the internet, and (if you need it) traffic leaves the server from that address. These checks take a few minutes and prevent hard-to-find problems later.

1. Is the address configured?

ip -br address
ip -4 address show

Every address you added must be in the list. On Windows use ipconfig /all.

2. Does the address answer from outside?

Run these from your own computer or another server, not from the server itself:

ping -c 3 203.0.113.11
nc -vz 203.0.113.11 22

The ping needs a firewall that allows ICMP; the nc command tests a real port (use one that a service on that address listens on). If both fail, check the firewall (How to write firewall rules for a server with many IP addresses?) and the routing information you received for the block (How to order an additional IPv4 subnet for your dedicated server?).

3. Which service listens on which address?

sudo ss -ltnp

The column Local Address shows the address. 0.0.0.0 means all addresses of the server, 203.0.113.11:443 means only that address.

4. Which address is used for outgoing connections?

By default a server uses its main address to connect to the internet. Ask an outside service which address it sees:

curl -s https://ifconfig.me; echo
curl -s --interface 203.0.113.11 https://ifconfig.me; echo

The first command shows the default outgoing address. The second forces the extra address and must print that address. If it prints your main address or fails, the extra address is not routed for outgoing traffic yet. Every additional subnet has its own gateway, so check that the routing table and the routing rule for that subnet exist: ip rule show, ip route show table 100 (use your table number) and ip route get 8.8.8.8 from 198.51.100.10, which shows the route that traffic from that address takes. The setup is in How to add additional IPv4 addresses on Ubuntu with netplan? and How to add additional IPv4 addresses on AlmaLinux or Rocky Linux with nmcli?. Ways to send from a chosen address are in How to choose the outgoing (source) IP address for a service, script or website?.

5. Test many addresses at once

for ip in 203.0.113.11 203.0.113.12 203.0.113.13; do
  echo -n "$ip -> "
  curl -s --max-time 10 --interface $ip https://ifconfig.me || echo "FAILED"
  echo
done

6. Reverse DNS

dig -x 203.0.113.11 +short

Should show the host name you set: How to set reverse DNS (PTR) for many IP addresses and check forward-confirmed rDNS?.

Common problems

  • The address is listed but never answers: wrong prefix length, wrong routing assumption or a firewall rule.
  • Outgoing test shows the main address: normal until you choose the source address for the service (How to choose the outgoing (source) IP address for a service, script or website?).
  • Works for a while, then stops: the address is missing from the permanent configuration. Check that you edited the netplan file or used nmcli connection modify, not only ip address add.

Frequently asked questions

Can I test from the server itself?
Pinging an address that is on the server always works, even when it is not reachable from the internet. Test from outside.

Which website can I use for the outgoing test?
Any "what is my IP" service that returns plain text will do; the one above is only an example.

Need a dedicated server, more IP addresses, or a hand with the setup?

Prefer a hand with the setup? Our engineers can do it for you: Hire an Expert, or use our on-demand server management.

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