UDP and TCP use internal software ports to support multiple conversations between various network devices. To differentiate the segments and datagrams for each application, TCP and UDP have header fields that uniquely identify these applications. These unique identifiers are the port numbers.
UDP 與 TCP 使用內部軟體連接埠，支援各種網路裝置之間多個對話的進行。為了區分各應用程式的區段與資料包，TCP 與 UDP 都有標頭欄位能唯一識別這些應用程式，這些唯一識別碼就是連接埠號。
Some of the applications that TCP/IP supports include:
TCP/IP 支援的部分應用程式包括：
- FTP (port 21, TCP): FTP is a reliable, connection-oriented service that uses TCP to transfer files between systems that support FTP. FTP supports bidirectional binary and ASCII file transfers. Besides using port 21 for exchange of control, FTP also uses one additional port, 20, for data transmission.FTP（連接埠 21，TCP）：FTP 是一種可靠的連線導向服務，使用 TCP 在支援 FTP 的系統之間傳輸檔案。FTP 支援雙向的二進位與 ASCII 檔案傳輸。除了使用連接埠 21 交換控制資訊外，FTP 還使用另一個連接埠 20 進行資料傳輸。
- SSH (port 22, TCP): Secure Shell (SSH) provides the capability to access other computers, servers, and networking devices remotely. SSH enables a user to log in to a remote host and execute commands. SSH messages are encrypted.SSH（連接埠 22，TCP）：安全殼層（SSH）提供遠端存取其他電腦、伺服器與網路裝置的能力。SSH 讓使用者能夠登入遠端主機並執行命令，SSH 訊息會經過加密。
- Telnet (port 23, TCP): Telnet is a predecessor to SSH. It sends messages in unencrypted cleartext. As a security best practice, most organizations now use SSH for remote communications.Telnet（連接埠 23，TCP）：Telnet 是 SSH 的前身，它以未加密的明文傳送訊息。基於安全最佳實務，大多數組織現在都改用 SSH 進行遠端通訊。
- HTTP (port 80, TCP): HTTP defines how messages are formatted and transmitted and which actions browsers and web servers can take in response to various commands. It uses TCP.HTTP（連接埠 80，TCP）：HTTP 定義了訊息的格式化與傳輸方式，以及瀏覽器與網頁伺服器針對各種命令可採取的動作，它使用 TCP。
- HTTPS (port 443, TCP): HTTPS combines HTTP with a security protocol (Secure Sockets Layer [SSL]/Transport Layer Security[TLS]).HTTPS（連接埠 443，TCP）：HTTPS 將 HTTP 與安全協定（安全通訊端層〔SSL〕／傳輸層安全性〔TLS〕）結合在一起。
- DNS (port 53, TCP,and UDP): DNS is used to resolve Internet names to IP addresses. DNS uses a distributed set of servers to resolve names that are associated with numbered addresses. DNS uses TCP for zone transfer between DNS servers and UDP for name queries.DNS（連接埠 53，TCP，與 UDP）：DNS 用來將網際網路名稱解析為 IP 位址。DNS 使用一組分散式伺服器，解析與編號位址對應的名稱。DNS 在 DNS 伺服器之間的區域傳輸使用 TCP，名稱查詢則使用 UDP。
- TFTP (port 69, UDP): TFTP is a connectionless service. Routers and switches use TFTP to transfer configuration files and Cisco IOS images, and other files between systems that support TFTP. TFTP is kind of an exception because it is a File Transfer Protocol but it uses UDP as a transport protocol. Recall that UDP is unreliable. If some segments are lost, the retransmission of the lost segments is not handled by the Layer 4 protocol but is handled by the application itself.TFTP（連接埠 69，UDP）：TFTP 是一種無連線服務。路由器與交換器使用 TFTP 在支援 TFTP 的系統之間傳輸設定檔與 Cisco IOS 映像檔等其他檔案。TFTP 算是一個例外，因為它雖然是檔案傳輸協定，卻使用 UDP 作為傳輸協定。別忘了 UDP 是不可靠的，若有區段遺失，遺失區段的重傳並非由第 4 層協定處理，而是由應用程式本身處理。
- SNMP (port 161, UDP): SNMP facilitates the exchange of management information between network devices. SNMP enables network administrators to manage network performance, find and solve network problems, and plan for network growth.SNMP（連接埠 161，UDP）：SNMP 促成網路裝置之間管理資訊的交換，讓網路管理員能夠管理網路效能、找出並解決網路問題，並規劃網路成長。
Here, you have seen only some applications with their port numbers. Go to the Service Name and Transport Protocol Port Number Registry for a complete list at http://www.iana.org/assignments/service-names-port-numbers/service-names-port-numbers.xhtml.
在此，你只看到部分應用程式及其連接埠號。完整清單請至服務名稱與傳輸協定連接埠號登錄查詢，網址為 http://www.iana.org/assignments/service-names-port-numbers/service-names-port-numbers.xhtml。