Windows 7 / Networking

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Default Port Assignments for Common Services and Tasks

Table below lists port numbers for common applications. If you are not sure which port numbers your application uses, consult the application's manual or contact the technical support team. Alternatively, you can use a protocol analyzer, such as Network Monitor, to examine network traffic to determine the port numbers used.

Service Name or Task	UDP	TCP
Web servers, HTTP, and Internet Information Services (IIS)		80
HTTP- Secure Sockets Layer (SSL)		443
DNS client-to-server lookup (varies)	53	53
DHCP client		67
File and printer sharing	137	139,445
FTP-control		21
FTP-data		20
Internet Relay Chat (IRC)		6667
Microsoft Office Outlook (see POP3, IMAP, and SMTP for ports)
Internet Mail Access Protocol (IMAP)		143
IMAP (SSL)		993
LDAP		389
LDAP (SSL)		636
Message Transfer Agent (MTA) - X.400 over TCP/IP		102
POP3		110
POP3 (SSL)		995
RPC endpoint mapper		135
SMTP		25
Network News Transfer Protocol (NNTP)		119
NNTP (SSL)		563
POP3		110
POP3 (SSL)		995
SNMP	161
SNMP Trap	162
SQL Server		1433
Telnet		23
Terminal Server and Remote Desktop		3389
Point-to-Point Tunneling Protocol (PPTP)		1723
Joining an AD DS domain (See the section titled "How to Troubleshoot Joining or Logging on to a Domain" later in this tutorial for more information.)

After identifying the port number, the first step in troubleshooting the application connectivity problem is to determine whether communications are successful using that port.

If it is a TCP port, you can use PortQry, Test TCP, or Telnet. Of those three tools, Telnet is the least flexible, but it is the only tool included with Windows (but note that it is not installed by default). For more information about Telnet, including how to install it, see the section titled "Telnet Client" earlier in this tutorial.

To test a TCP port with Telnet, run the following command.

Telnet hostname_or_address TCP_port

For example, to determine whether you can connect to the Web server at www.sourcedaddy.com (which uses port 80), you would run the following command.

Telnet www.sourcedaddy.com 80

If the command prompt clears or if you receive text from the remote service, you have successfully established a connection. Close the command prompt to cancel Telnet. This indicates that you can connect to the server; therefore, the server application is listening for incoming connections and no firewall is blocking your traffic. Instead of troubleshooting the problem as a connectivity issue, you should consider application-level issues, including:

• Authentication issues View the server's Security Event Log or the application's log to determine whether it is rejecting your client connections because of invalid credentials.
• Failed service Restart the server. Test whether other client computers can connect to the server.
• Invalid client software Verify that the client software running on your computer is the correct version and is configured properly.

If Telnet displays "Could not open connection to the host," this indicates an application connectivity issue, such as a misconfigured firewall. Follow these steps to continue troubleshooting the problem:

1. If possible, verify that the server is online. If the server is online, attempt to connect to a different service running on the same server. For example, if you are attempting to connect to a Web server and you know that the server has file sharing enabled, attempt to connect to a shared folder. If you can connect to a different service, the problem is almost certainly a firewall configuration problem on the server.
2. Attempt to connect from different client computers on the same and different subnets. If you can connect from a client computer on the same subnet, you might have an application configuration problem on the client computer. If you can connect from a client computer on a different subnet but not from the same subnet, a firewall on the network or on the server might be filtering traffic from your client network.
3. If possible, connect a client computer to the same subnet as the server. If you can connect from the same subnet but not from different subnets, a router-based firewall is blocking traffic. If you cannot connect from the same subnet, the server has a firewall that is blocking traffic. Alternatively, the server application might not be running or might be configured to use a different port.
4. Log on to the server and use Telnet to attempt to connect to the server application port. If you can connect to the server from the server but not from other computers, the server definitely has firewall software configured. Add an exception for the application to the firewall software. If you cannot connect to the server application from the server, the application is not listening for connections or is configured to listen for incoming connections on a different port. Refer to the application documentation for information on how to start and configure the application. If the server is running Windows, you can use Netstat to identify on which ports the server is listening for incoming connections. For more information, read the section titled "Netstat" earlier in this tutorial.