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The short definitions
- IP address: identifies a host on a network.
- Port: identifies which application or service on that host should receive the traffic. TCP and UDP use 16-bit port numbers, so the range is 0 to 65535, or 65,536 values (Oracle’s Java networking tutorial).
- Socket: a software endpoint tied to an address and a port. In real socket programming it also carries a transport protocol context.
- Connection: a conversation between two endpoints. RFC 793, the foundational TCP specification, says: “A connection is fully specified by the pair of sockets at the ends.” It also notes that a connection can carry data in both directions (full duplex).
Oracle’s tutorial puts the role of ports this way: “The TCP and UDP protocols use ports to map incoming data to a particular process running on a computer.”
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Port vs. socket vs. connection
The three terms are related but not interchangeable.
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|---|---|---|
| Port | A service or process slot on one host, interpreted with a protocol | TCP 443 |
| Socket | One endpoint: host address plus port | 203.0.113.10:443 |
| Connection (TCP) | A pair of sockets, one at each end | 198.51.100.7:52114 ↔ 203.0.113.10:443 |
The addresses in the example are reserved documentation addresses, used here only for illustration.
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How one server handles many connections
NIST SP 800-10, a legacy publication, explains that a TCP or UDP connection is uniquely identified by four values: source IP address, destination IP address, source port and destination port. This is what lets a single web server listening on one port hold thousands of conversations at once. The destination port is the same for every client, but each client’s address and source port differ, so every combination is distinct.
Two laptops connecting to the same HTTPS site both target the server’s TCP port 443. Even if both happened to pick the same source port, their IP addresses would still tell the connections apart. The same laptop opening two tabs to that site uses two different source ports.
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Service ports and ephemeral ports
Service (server-side) ports
A server process binds to a known port and listens so clients know where to send requests. Microsoft defines a service port as the port on which a system service listens. Common examples, as used in Ubiquiti’s networking guide, are HTTP on TCP 80 and HTTPS on TCP 443.
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For an outgoing connection, the client’s operating system normally assigns a temporary local source port. Microsoft documents a default dynamic client range of 49152–65535 for the Windows configuration described in its port-exhaustion troubleshooting article. That figure is Windows-specific and configuration-dependent; other systems and settings use different ranges. Microsoft also warns that exhausting these ports can stop new outbound connections from being made.
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Port number ranges
IANA, which maintains the official registry, divides port numbers into three categories:
| Category | Range |
|---|---|
| System Ports | 0–1023 |
| User Ports | 1024–49151 |
| Dynamic and/or Private Ports | 49152–65535 |
The registry result reviewed showed a last-updated date of 2026-09-30. Because assignments change, check the live IANA Service Name and Transport Protocol Port Number Registry when you need the exact current assignment, and treat old example tables as illustrative rather than exhaustive.
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TCP and UDP ports
TCP is connection-oriented: it sets up a connection and provides reliable, ordered delivery. UDP sends independent datagrams without TCP’s connection setup or delivery guarantees. Ubiquiti presents TCP as typical for web browsing and email, and UDP as typical for real-time uses such as gaming, video and voice. These are broad tendencies, not rules, since applications pick transports for their own reasons.
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A port number only has meaning alongside its protocol. TCP 53 and UDP 53 are separate endpoints that are not automatically the same service, and IANA registers service names against specific transports. Say “TCP 443,” not just “port 443,” when the difference matters. Also, RFC 793’s definition of a connection is specific to TCP. Do not assume UDP has the same connection model.
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Listening, open and reachable
People use “open port” loosely, and it combines two separate conditions:
- Listening: a service is bound to the port and ready to receive.
- Open (informal): traffic can actually reach that port through the network path and firewall policy.
Neither implies the other. A firewall rule does not create a service, and a listening service is not reachable if packets are blocked on the way. So a port number alone never proves connectivity.
Physical ports are a different thing
An Ethernet jack, USB socket or HDMI connector is a hardware port, a physical interface for cables. Network ports are numbers inside the protocol stack. Plugging in a cable gives you a physical link, but it does not open, close or change any TCP or UDP port.
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