Re: Issue with some unnamed ISP's in northern north america, RFC 1918 Violation

[Laszlo H via NANOG <nanog () lists nanog org>]

Not sure if you meant Comcast but it sounds similar to what they do.  I 
think they are using RIP not BGP, but it works the way you're 
describing.  You have to statically configure the static IP and gateway 
- if you let it DHCP it will get one of those 10.x.x.x numbers.  It's 
just two addresses on the same interface on the CPE: the 10.x.x.1 IP and 
also the first address from your static IP block.


On 3/10/26 9:13 PM, wolf1098--- via NANOG wrote:
> One of the fibre providers, they are issuing static ips, which is good, but the way the statics are handled. The modem/router, if you want static 
> ips, requires it to be in router mode, you can't bridge. it seems the router's static ip gateway does some bgp towards the isp to route the 
> static ip's block assigned to the customer to it, via its dhcp established route??? idk. ------ THE big issue, which is, since it has to be in 
> router mode. they end up with a dynamic ip, that is the gateway for dhcp RFC1918 addresses, that are on its lan, sounds reasonable, but as a tertiary 
> use, the static ip WAN gw stack, LOADS IN ON the same L2 lan bridge, side by side with the RFC 1918 space, without even attempting to put it in a 
> tagged vlan. don't do this.... without using a custom linux router, good luck using both the static ips block, and the double 'natted' 
> dynamic ip.
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