Port 139 (NetBIOS-SSN) — What It Is and Security [2026]

Enterno.io Editorial Team

Port 139: NetBIOS-SSN — Complete Guide

By Anatoly Oshmanovsky · Updated Apr 17, 2026

TL;DR:

Legacy Windows protocol. Replaced by SMB over 445. Close on the internet. Standard TCP port for NetBIOS-SSN.

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What is port 139

Port 139 is reserved for NetBIOS-SSN. Legacy Windows protocol. Replaced by SMB over 445. Close on the internet.

Check port openness

Check port 139 →

Security

Close the port if the service isn't used. For admin ports, require IP whitelist or VPN. Monitor connection attempts via fail2ban.

ICMP PingHost availability and latency

Port ScannerOpen TCP port detection

LatencyResponse time in milliseconds

Packet LossPercentage of dropped packets

Why teams trust us

ICMP+TCP

check protocols

key ports scanned

<2s

result

regions

How it works

Enter IP or domain

ICMP packets sent

Latency & packet loss shown

How Do Ping and Port Scanning Work?

Ping sends ICMP packets to a host and measures response time. Port scanning checks which TCP ports are open and accepting connections — helping diagnose serviceavailability issues.

Configurable Ping

Choose packet count (3, 4, 6, 10). Stats: min/avg/max latency and packet loss.

Common Port Scanner

Check 14 key ports: HTTP, HTTPS, SSH, FTP, SMTP, MySQL, PostgreSQL, and more.

Cloud-Based Check

Testing from our server — see site availability from outside, not just your local network.

Uptime Monitoring

Need constant monitoring? Create a monitor — checks every minute with notifications.

Who uses this

DevOps

availability diagnosis

Network engineers

TCP port scanning

Developers

connection debugging

SRE

basic health check

Common Mistakes

❌

ICMP blocked = server is downMany servers block ICMP. Ping fails but site works — check ports instead.

❌

High ping = server problemLatency depends on geography. 150ms between continents is normal, not an error.

❌

Closed ports — cause for alarmClosed ports of unused services are good. Unnecessary open ports are a risk.

❌

One check = sufficientNetworks are unstable. A single timeout ≠ a problem. Check multiple times or set up monitoring.

Best Practices

✓

Combine ping and port checksPing shows host availability, ports show specific service availability. Use both.

✓

Check from different locationsThe problem may be local. A cloud test shows the real picture.

✓

Close unused portsEvery open port is a potential attack vector. Keep only necessary ports open.

✓

Set up monitoringManual checks do not scale. Set up automated monitoring with notifications.

Get more with a free account

Ping check history, host availability monitoring and downtime alerts.

Learn more

Research

Open Port Exposure in Runet 2026

Alternatives

Frequently Asked Questions

Why close port 139?

Every open port is a potential entry point. If the service isn't used — close it.