Network Tools

Port Checker

Check if TCP ports are open on any host or IP. Test up to 10 ports at once and see response latency — useful for firewall and server debugging.

TCP
Up to 10 Ports
Latency
Open / Closed Status
Instant

Host

Ports

Comma-separated, up to 10

Enter host and ports to check

Understanding Network Ports & Port Checking

In computer networking, a port is a 16-bit unsigned integer (ranging from 0 to 65535) associated with an IP address to direct incoming and outgoing packets to specific application processes. Port checking tests whether a specific port on a server or host is open, closed, or filtered by a firewall.

Port Ranges & Allocations

Standard port numbers are coordinated globally by the Internet Assigned Numbers Authority (IANA):

Range	Port Category	Description
`0 - 1023`	Well-Known Ports	Reserved for core internet services (HTTP, HTTPS, SSH, DNS)
`1024 - 49151`	Registered Ports	Assigned to proprietary user and application services (MySQL, Redis, MongoDB)
`49152 - 65535`	Dynamic / Ephemeral Ports	Allocated dynamically by operating systems for outbound client sockets

Common Well-Known Service Ports

Port	Protocol	Default Service	Function
21	TCP	FTP	File Transfer Protocol control channel
22	TCP	SSH	Secure Shell remote server administration
53	TCP/UDP	DNS	Domain Name System name resolution
80	TCP	HTTP	Unencrypted web traffic
443	TCP	HTTPS	TLS/SSL encrypted web traffic
3306	TCP	MySQL	MySQL Database server socket
5432	TCP	PostgreSQL	PostgreSQL Database server socket
6379	TCP	Redis	Redis in-memory cache daemon

Diagnostic Port Scanning Tools

To diagnose port accessibility from a development workstation or server:

# Test specific TCP port with netcat
nc -zv besttoolonline.com 443

# Test with curl (shows handshake speed)
curl -v telnet://besttoolonline.com:443

# Check open listening ports on Linux/macOS host
ss -tuln
# or
netstat -tulpn

Firewalls, NAT & Port Forwarding Fundamentals

Network Address Translation (NAT): Home and enterprise routers hide internal device IPs behind a single public IP. External connections to internal services require explicit Port Forwarding rules.
Stateful Packet Inspection (SPI): Modern firewalls track ongoing TCP connection states, automatically permitting return packets for outbound connections while blocking unsolicited inbound probes.
Security Audits: Routine external port checks ensure developmental services (like local Redis, Docker daemons, or MongoDB instances) are not unintentionally bound to 0.0.0.0 on public network interfaces.

Frequently Asked Questions

What does it mean if a network port is filtered versus closed?: A closed port indicates the host is reachable and responded with a TCP RST packet confirming no service is listening. A filtered port indicates packets were dropped silently by a firewall or router without any response, preventing an observer from determining if the port is active.
Why is scanning ports important for server security audits?: Port auditing helps system administrators identify unintentional listening daemons, exposed database ports, and unpatched network services. Closing unused ports significantly reduces attack surfaces against automated botnet probes.
Can I test UDP ports with standard TCP checkers?: No. TCP relies on three-way handshakes (SYN, SYN-ACK, ACK) to confirm connections, while UDP is a connectionless protocol that does not acknowledge incoming datagrams unless the application specifically responds.
What does a port checker do?: A port checker attempts a TCP connection to a specific port on a host. If the connection succeeds, the port is open; if it is refused or times out, the port is closed or filtered.
What is a port number?: Port numbers (0–65535) identify specific services on a network host. Common ports: 80 (HTTP), 443 (HTTPS), 22 (SSH), 3306 (MySQL), 5432 (PostgreSQL).
Why might a port appear closed when it is open?: Firewalls, NAT, security groups, or load balancers may block external connections. The port may be open internally but closed to external traffic.
Can I check multiple ports at once?: Yes. Enter a comma-separated list of ports (e.g. 80,443,22) to check them all in one request.

Accuracy & Advisory Notice: Port Checker

This website and utility are provided for general informational, calculation, and educational purposes only. Always verify critical computation outputs prior to professional deployment or financial use. See our Terms of Service and Privacy Policy for full governance details.

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Understanding Network Ports & Port Checking

Port Ranges & Allocations

Standard port numbers are coordinated globally by the Internet Assigned Numbers Authority (IANA):

Range

Port Category

Description

0 - 1023

Well-Known Ports

Reserved for core internet services (HTTP, HTTPS, SSH, DNS)

1024 - 49151

Registered Ports

Assigned to proprietary user and application services (MySQL, Redis, MongoDB)

49152 - 65535

Dynamic / Ephemeral Ports

Allocated dynamically by operating systems for outbound client sockets

Common Well-Known Service Ports

Port

Protocol

Default Service

Function

TCP

FTP

File Transfer Protocol control channel

TCP

SSH

Secure Shell remote server administration

TCP/UDP

DNS

Domain Name System name resolution

TCP

HTTP

Unencrypted web traffic

443

TCP

HTTPS

TLS/SSL encrypted web traffic

3306

TCP

MySQL

MySQL Database server socket

5432

TCP

PostgreSQL

PostgreSQL Database server socket

6379

TCP

Redis

Redis in-memory cache daemon

Diagnostic Port Scanning Tools

To diagnose port accessibility from a development workstation or server:

# Test specific TCP port with netcat nc -zv besttoolonline.com 443 # Test with curl (shows handshake speed) curl -v telnet://besttoolonline.com:443 # Check open listening ports on Linux/macOS host ss -tuln # or netstat -tulpn

Firewalls, NAT & Port Forwarding Fundamentals

Network Address Translation (NAT): Home and enterprise routers hide internal device IPs behind a single public IP. External connections to internal services require explicit Port Forwarding rules.

Stateful Packet Inspection (SPI): Modern firewalls track ongoing TCP connection states, automatically permitting return packets for outbound connections while blocking unsolicited inbound probes.

Security Audits: Routine external port checks ensure developmental services (like local Redis, Docker daemons, or MongoDB instances) are not unintentionally bound to 0.0.0.0 on public network interfaces.

Frequently Asked Questions

What does it mean if a network port is filtered versus closed?

A closed port indicates the host is reachable and responded with a TCP RST packet confirming no service is listening. A filtered port indicates packets were dropped silently by a firewall or router without any response, preventing an observer from determining if the port is active.

Why is scanning ports important for server security audits?

Port auditing helps system administrators identify unintentional listening daemons, exposed database ports, and unpatched network services. Closing unused ports significantly reduces attack surfaces against automated botnet probes.

Can I test UDP ports with standard TCP checkers?

No. TCP relies on three-way handshakes (SYN, SYN-ACK, ACK) to confirm connections, while UDP is a connectionless protocol that does not acknowledge incoming datagrams unless the application specifically responds.

What does a port checker do?

A port checker attempts a TCP connection to a specific port on a host. If the connection succeeds, the port is open; if it is refused or times out, the port is closed or filtered.

What is a port number?

Port numbers (0–65535) identify specific services on a network host. Common ports: 80 (HTTP), 443 (HTTPS), 22 (SSH), 3306 (MySQL), 5432 (PostgreSQL).

Why might a port appear closed when it is open?

Firewalls, NAT, security groups, or load balancers may block external connections. The port may be open internally but closed to external traffic.

Can I check multiple ports at once?

Yes. Enter a comma-separated list of ports (e.g. 80,443,22) to check them all in one request.