Interface-based source NAT is the most appropriate and efficient solution for a Juniper SRX Series device when only a single public IP address is available from the ISP. This configuration method instructs the SRX to use the IP address of its egress (external-facing) interface as the source address for all translated traffic originating from the internal network. This method implicitly uses Port Address Translation (PAT) to allow multiple internal hosts to share the single public IP address simultaneously by mapping each session to a unique port number. It is the standard and simplest configuration for this common scenario.

A. pool-based NAT with PAT: While a pool could be configured with a single IP, "interface-based" NAT is the more direct and specific Juniper feature designed for using the egress interface's IP address.

B. pool-based NAT with address shifting: This is a form of static NAT that requires a one-to-one mapping between private and public IP addresses, making it unsuitable for sharing a single IP among many hosts.

D. pool-based NAT without PAT: Without PAT, a single public IP could only be used by one internal host at a time, as there would be no mechanism to differentiate between concurrent sessions.

1. Juniper Networks TechLibrary

"Understanding Source NAT." This document explicitly describes the two types of source NAT: interface source NAT and source NAT with an address pool. It states

"With interface source NAT

the IP address of the egress interface is used as the source address for the translated traffic." This directly matches the scenario in the question.

URL: https://www.juniper.net/documentation/us/en/software/junos/nat/topics/concept/nat-source-understanding.html

2. Juniper Networks TechLibrary

"Example: Configuring Source NAT (Interface)." This configuration example demonstrates the exact use case. The overview states

"In this example

you configure source NAT to use the IP address of the egress interface as the translation address."

URL: https://www.juniper.net/documentation/us/en/software/junos/nat/topics/example/nat-source-interface-configuring.html

3. Juniper Networks TechLibrary

"Source NAT Rule Configuration Overview." This guide details the source-nat stanza

showing the distinct interface and pool options

reinforcing that "interface" is the specific choice when using the egress interface's IP.

URL: https://www.juniper.net/documentation/us/en/software/junos/nat/topics/topic-map/security-source-nat-rules.html (See the section on Rule Action).