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Port 5421 (netsupport2)

IANA assigns port 5421 to netsupport2: Net Support 2.

Registered (user) port, 1024-49151 TCPUDP

IANA assignment

Service nameProtoDescriptionAssigneeReference
netsupport2 TCP Net Support 2 Paul Sanders -
netsupport2 UDP Net Support 2 Paul Sanders -

Check what is listening on port 5421

Linux: is anything listening on TCP 5421?
sudo ss -tlnp 'sport = :5421'
Linux: UDP 5421
sudo ss -ulnp 'sport = :5421'
Linux / macOS: which process owns the port
sudo lsof -nP -i :5421
Older Linux / BSD (net-tools)
sudo netstat -tlnp | grep ':5421 '
Windows (Command Prompt)
netstat -ano | findstr :5421
Windows (PowerShell)
Get-NetTCPConnection -LocalPort 5421 -State Listen | Select LocalAddress,OwningProcess

Test it from outside

From another machine
nc -vz -w 5 HOST 5421
From Windows
Test-NetConnection HOST -Port 5421
UDP probe (no reply does not prove it is closed)
nc -vzu -w 5 HOST 5421

Block port 5421 in the firewall

ufw
sudo ufw deny 5421/tcp
iptables
sudo iptables -A INPUT -p tcp --dport 5421 -j DROP
nftables
sudo nft add rule inet filter input tcp dport 5421 drop
Windows Firewall
New-NetFirewallRule -DisplayName 'Block TCP 5421' -Direction Inbound -Protocol TCP -LocalPort 5421 -Action Block
ufw
sudo ufw deny 5421/udp
iptables
sudo iptables -A INPUT -p udp --dport 5421 -j DROP
nftables
sudo nft add rule inet filter input udp dport 5421 drop
Windows Firewall
New-NetFirewallRule -DisplayName 'Block UDP 5421' -Direction Inbound -Protocol UDP -LocalPort 5421 -Action Block

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All port numbers · Open port checker
Assignment data: IANA port number registry (public domain), retrieved 2026-11-10.