Port 65535 Highest port · TCP and UDP

What runs on port 65535

The highest possible port number (2¹⁶ − 1) and the end of the ephemeral range. Nothing standard listens here; it appears in firewall rules as the upper bound of ranges and occasionally as a "hidden" port chosen by someone who thought scanners stop earlier (they do not).

Software that uses it: none by convention

Should port 65535 be open?

Private network or VPN only. Nothing good comes from this port being reachable from the internet. Keep it on an internal network, reach it over a VPN or an SSH tunnel, and firewall it from everything that does not need it.

• Port numbers are 16-bit unsigned: 0–65535. Port 0 is reserved and means "pick one for me" in socket APIs.

Check port 65535

# on the host: is anything listening, and which process?
ss -ltnp 'sport = :65535'
ss -lunp 'sport = :65535'
sudo lsof -i :65535

# from another machine: is it reachable?
nc -zv host 65535
nmap -p 65535 host
nc -zuv host 65535
sudo nmap -sU -p 65535 host
curl -v telnet://host:65535      # raw TCP connect test, Ctrl-C to leave

A port that answers from the internet when it should not is the finding; nmap from a machine outside your network (or a VPS) is the honest test, because scans from inside the LAN pass the firewall you are trying to verify.

Allow or block port 65535

# ufw (Ubuntu/Debian)
sudo ufw allow from 10.0.0.0/8 to any port 65535 proto tcp
sudo ufw allow from 10.0.0.0/8 to any port 65535 proto udp
sudo ufw deny 65535/tcp
sudo ufw deny 65535/udp

# nftables
nft add rule inet filter input ip saddr 10.0.0.0/8 meta l4proto { tcp, udp } th dport 65535 accept
nft add rule inet filter input meta l4proto { tcp, udp } th dport 65535 drop

# iptables (legacy)
iptables -A INPUT -p tcp --dport 65535 -s 10.0.0.0/8 -j ACCEPT
iptables -A INPUT -p tcp --dport 65535 -j DROP
iptables -A INPUT -p udp --dport 65535 -s 10.0.0.0/8 -j ACCEPT
iptables -A INPUT -p udp --dport 65535 -j DROP

# firewalld
sudo firewall-cmd --permanent --add-rich-rule='rule family=ipv4 source address=10.0.0.0/8 port port=65535 protocol=tcp accept'

Replace 10.0.0.0/8 with the networks that need access. Allowing by source address is the rule that matters; "allow 65535" with no source is just "open".

Related ports

• 49152 Dynamic / ephemeral range starts — IANA ephemeral ports 49152–65535 (Linux uses 32768–60999)

Part of the port number reference. Ports are 16-bit numbers; see also the subnet calculator and the packet decoder, which shows ports inside TCP and UDP headers.

Questions