Domain Names/Static IP
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Static Block and Gateway Question

We've been paying for a block of five static IPs since 2006, but have always used just one. We now have a situation that requires an additional IP, but our modem is configured with a CIDR /30, which yields only one usable IP. If we switch to /29 we'll have all five IPs, but the extra four addresses will be above what is now our gateway. It's standard practice for the highest IP in a block to be the gateway, but is it technically possible to use a gateway in the middle of the range?

 

My reason for asking is that our firewall/router/vpn is managed by our corporate guys and something as simple as changing the gateway can become an ordeal. Mix that with coordinating with Comcast, and the odds of "downtime out the wazoo" are unacceptable.

 

Any way to expand to the full block of IPs we're paying for without changing my gateway?

 

Thanks!

 

D.

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Community Manager
Community Manager

Re: Static Block and Gateway Question

Welcome DC_LR.  Using a gateway IP that is in the middle of the range will cause routing issues with IP address above the gateway IP address.  Also when changed from CIDR /30 to CIDR /29 you will be issue a new IP scope and since that scope is not an expansion of your current /30 it is necessary to change your network to match the new CIDR /29 IP scope.  

 

Thank You

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Re: Static Block and Gateway Question

Routing is like Honey Badger, it don't give a .....

From a strict routing perspective, the router can be anywhere as long as it's a valid IP in the network.  Keep in mind that the 1st IP (all 0s in binary) is the network address, and the last IP (all 1s in binary) is the broadcast).  You can't use those in any netblock.

For a example /29: 10.123.45.128/29

10.123.45.128 = network address.  The last 3 bits are all 0s (29 bits are subnet, see where we get /29?)

10.123.45.129 - 134 are usable for devices.  6 in total, but you gotta have a router, so that's where 5 comes from.

10.123.45.135 = broadcast address.  The last 3 bits are all 1s.

That all said, it's possible that the router Comcast provides would have issues being inside a network.  I think it's unlikely, but it's worth testing I think.  The router is going to go through the same math for each packet in any case (it's a simple bitwise AND and compare calculation to see what to do with a packet).  The location of the router doesn't change the math at all.

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Community Manager
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Re: Static Block and Gateway Question

Welcome DC_LR.  Using a gateway IP that is in the middle of the range will cause routing issues with IP address above the gateway IP address.  Also when changed from CIDR /30 to CIDR /29 you will be issue a new IP scope and since that scope is not an expansion of your current /30 it is necessary to change your network to match the new CIDR /29 IP scope.  

 

Thank You

View solution in original post

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Visitor

Re: Static Block and Gateway Question

Routing is like Honey Badger, it don't give a .....

From a strict routing perspective, the router can be anywhere as long as it's a valid IP in the network.  Keep in mind that the 1st IP (all 0s in binary) is the network address, and the last IP (all 1s in binary) is the broadcast).  You can't use those in any netblock.

For a example /29: 10.123.45.128/29

10.123.45.128 = network address.  The last 3 bits are all 0s (29 bits are subnet, see where we get /29?)

10.123.45.129 - 134 are usable for devices.  6 in total, but you gotta have a router, so that's where 5 comes from.

10.123.45.135 = broadcast address.  The last 3 bits are all 1s.

That all said, it's possible that the router Comcast provides would have issues being inside a network.  I think it's unlikely, but it's worth testing I think.  The router is going to go through the same math for each packet in any case (it's a simple bitwise AND and compare calculation to see what to do with a packet).  The location of the router doesn't change the math at all.

View solution in original post

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