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So, can I set 127.0.0.1 as the default gateway to my router/modem instead 192.168.1.1? If yes, why?

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    Why on earth would you want to do that? – Ron Trunk Apr 3 '17 at 13:22
  • But can be setted in that way? – CivilEngine Apr 3 '17 at 13:23
  • It depends on the router model. Some products may not let you. But again I have to ask, what are you trying to do? – Ron Trunk Apr 3 '17 at 13:25
  • it was just a personal doubt – CivilEngine Apr 3 '17 at 13:38
  • Did any answer help you? If so, you should accept the answer so that the question doesn't keep popping up forever, looking for an answer. Alternatively, you can provide and accept your own answer. – Ron Maupin Feb 19 '18 at 2:59
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No. Any address in the 127.0.0.0/8 block can never appear anywhere on any network, nor can any address in that block be used as a source or destination address.

The goes back at least as far as RFC 990, ASSIGNED NUMBERS:

The class A network number 127 is assigned the "loopback" function, that is, a datagram sent by a higher level protocol to a network 127 address should loop back inside the host. No datagram "sent" to a network 127 address should ever appear on any network anywhere.

RFC 1122, Requirements for Internet Hosts -- Communication Layers:

(g) { 127, }

Internal host loopback address. Addresses of this form MUST NOT appear outside a host.

Also RFC 3330, Special-Use IPv4 Addresses:

127.0.0.0/8 - This block is assigned for use as the Internet host loopback address. A datagram sent by a higher level protocol to an address anywhere within this block should loop back inside the host. This is ordinarily implemented using only 127.0.0.1/32 for loopback, but no addresses within this block should ever appear on any network anywhere [RFC1700, page 5].

By the way, routers that are actually routing shouldn't use the default-gateway command because they are the gateways. Instead, they should use a default route.

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