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mike18
Level 7
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Message 1 of 7

Inside and Outside interface on Same subnet

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Hi Everyone,

Seems one of firewall which we have ---- its having  same subnet for both inside and outside interface.

Does this make sense?

Normally in cisco firewalls i have seen different subnet for each interface?

Regards

Mike

1 Solution

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PhilM
Level 14
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Message 6 of 7

Re: Inside and Outside interface on Same subnet

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Mike -

I'm not sure you understand what Sam is saying. Each interface can have the same subnet mask value (/29, /24, etc...), but the interfaces themselves cannot be configured with the same address range.

Interface 1 - 1.1.1.1/24 & Interface 2 - 2.2.2.2/24 (both /24, but different address ranges) = OK

Interface 1 - 1.1.1.1/24 & Interface 2 - 1.1.1.2/24 (both part of the same address subnet) = Not OK.

-Phil.

6 Replies
sliedl
Level 14
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Message 2 of 7

Re: Inside and Outside interface on Same subnet

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You cannot have the exact same subnet on both interfaces.  You could have two that overlap and the most-specific one would take precedence.  I don't see any reason for setting the firewall up like that, though.

mike18
Level 7
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Message 3 of 7

Re: Inside and Outside interface on Same subnet

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I check both have same subnet /29.

sliedl
Level 14
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Message 4 of 7

Re: Inside and Outside interface on Same subnet

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If you mean "Can you have two interfaces with the same subnet mask?" then the answer is Yes, of course.  If you question is "Can you configure two interfaces with the same IP address and the same subnet mask?" then the answer is No.

mike18
Level 7
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Message 5 of 7

Re: Inside and Outside interface on Same subnet

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Seems in this environment  it  is working fine with same subnet mask /29 strange.

Under what circumstance we can have two interfaces with subnet ?

Regards

Mike

PhilM
Level 14
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Message 6 of 7

Re: Inside and Outside interface on Same subnet

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Mike -

I'm not sure you understand what Sam is saying. Each interface can have the same subnet mask value (/29, /24, etc...), but the interfaces themselves cannot be configured with the same address range.

Interface 1 - 1.1.1.1/24 & Interface 2 - 2.2.2.2/24 (both /24, but different address ranges) = OK

Interface 1 - 1.1.1.1/24 & Interface 2 - 1.1.1.2/24 (both part of the same address subnet) = Not OK.

-Phil.

mike18
Level 7
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Message 7 of 7

Re: Inside and Outside interface on Same subnet

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I got it now Phil

Best Regards

Mike

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