应用层阐道（英语：Application Layer Gateway，或application-level gateway，缩写为 ALG） 是一种NAT穿透技术。就应用层面来说，它允许修改匣道上的 NAT traversal 的过滤规则，完全某个应用程序在地址及通信端口上的转换。举例来说，像 FTP，BitTorrent，SIP，RTSP，或 IM 等即时通软件上的文件传递功能，这些都可以使用 ALG 来针对应用程序在地址及通信端口转换上的需求。在RFC 2663中定义了这个功能。

ALG 本身具有 SIP 能力（SIP-aware），但必须更换NAT。因此客户接受度普遍不高。为了广推ALG，又有所谓的 Middlebox communication（MIDCOM）协定，MIDCOM 允许应用程序控制NAT。

In the context of computer networking, an application-level gateway (also known as ALG or application layer gateway) consists of a security component that augments a firewall or NAT employed in a computer network. It allows customized NAT traversal filters to be plugged into the gateway to support address and port translation for certain application layer "control/data" protocols such as FTP, BitTorrent, SIP, RTSP, file transfer in IM applications, etc. In order for these protocols to work through NAT or a firewall, either the application has to know about an address/port number combination that allows incoming packets, or the NAT has to monitor the control traffic and open up port mappings (firewall pinhole) dynamically as required. Legitimate application data can thus be passed through the security checks of the firewall or NAT that would have otherwise restricted the traffic for not meeting its limited filter criteria.