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Port 101 (hostname)

IANA assigns port 101 to hostname: NIC Host Name Server.

Well-known (system) port, 0-1023 TCPUDP

IANA assignment

Service nameProtoDescriptionAssigneeReference
hostname TCP NIC Host Name Server Jon Postel -
hostname UDP NIC Host Name Server Jon Postel -

Check what is listening on port 101

Linux: is anything listening on TCP 101?
sudo ss -tlnp 'sport = :101'
Linux: UDP 101
sudo ss -ulnp 'sport = :101'
Linux / macOS: which process owns the port
sudo lsof -nP -i :101
Older Linux / BSD (net-tools)
sudo netstat -tlnp | grep ':101 '
Windows (Command Prompt)
netstat -ano | findstr :101
Windows (PowerShell)
Get-NetTCPConnection -LocalPort 101 -State Listen | Select LocalAddress,OwningProcess

Test it from outside

From another machine
nc -vz -w 5 HOST 101
From Windows
Test-NetConnection HOST -Port 101
UDP probe (no reply does not prove it is closed)
nc -vzu -w 5 HOST 101

Block port 101 in the firewall

ufw
sudo ufw deny 101/tcp
iptables
sudo iptables -A INPUT -p tcp --dport 101 -j DROP
nftables
sudo nft add rule inet filter input tcp dport 101 drop
Windows Firewall
New-NetFirewallRule -DisplayName 'Block TCP 101' -Direction Inbound -Protocol TCP -LocalPort 101 -Action Block
ufw
sudo ufw deny 101/udp
iptables
sudo iptables -A INPUT -p udp --dport 101 -j DROP
nftables
sudo nft add rule inet filter input udp dport 101 drop
Windows Firewall
New-NetFirewallRule -DisplayName 'Block UDP 101' -Direction Inbound -Protocol UDP -LocalPort 101 -Action Block

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Assignment data: IANA port number registry (public domain), retrieved 2026-11-10.