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I just got back from a site visit where I had to use a fixed IP, gateway, and DNS address. When off the main network there, I used a USB 3G modem with no problem, but as soon as I got home, I could not even connect to my Billion 7300 router/modem, let alone the internet, until I went and set the connection to dynamic IP address and gateway+dns.

Why is this?

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up vote 4 down vote accepted

The reason for this is simply DHCP (Dynamic Host Configuration Protocol), it is a method for automatically assigning IP/Netmask, Gateway, DNS and a variety of other settings - when you have not manually configured settings.

When you manually place IP settings, it basically uses them instead of querying DHCP servers.

When you plugged in your 3G modem, it would have set up its own device independently of other network connections as you can manually set a different address on each connection. It will itself most likely be using DHCP to your ISP's server or the installer would have manually set them.

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When you got home, obviously, you were connecting via the same connection as the site and their settings were taking preference over DHCP.

The way around this is to put the site settings as the "alternate configuration" and leave the main settings as DHCP, this will mean that you will always use DHCP but if it cannot find a DHCP server, it will use the settings present in the alternate configuration.

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If you need to switch frequently between various connections, have a look at this:

NetSetMan allows you to quickly switch between pre-configured network settings. The program is ideal for people on the road, who frequently connect to different networks and need to update or reconfigure their network settings each time. NetSetMan eliminates the need for manual configurations, you can simply create different profiles to store your network configuration, and then switch between them with the click of a button. NetSetMan can apply configuration settings for IP address Subnet Mask, Default Gateway, WiFi, Printer, DNS servers and more. The free version is limited to 6 network profiles, which is more than enough for personal usage - commercial usage requires a license.

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or this:

IP Organizer 2 does what it says on the tin, and it does it for free. pretty much like NetSetMan, it allows you to switch between multiple network profiles (unlimited). NetSetMan features more functions but is only free for private use while IP Organizer 2 is free as in free beer.

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The website is in Turkish, here's the download link.

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