[lug] networking question

[Zan Lynx]

On 9/12/2020 5:36 PM, David L. Willson wrote:
> This feels like a newbie question that should be obvious to me, but it's 
> not obvious to me, so I'll ask.
> 
> If I have a subnet behind my router, and I want to put *part* of that 
> subnet (a sub-subnet?) behind an interior router (sub-router?)... Can I 
> do that?
> 
> Example (the actual case in point):
> 
> I have 67.42.246.112/29. It routes through 67.42.246.126. I have control 
> of 67.42.246.126. It's not Linux, but it's not entirely brainless, either.
> 
> Is there a way for me to carve the upper or lower /28 (67.42.246.112/28 
> or 67.42.246.120/28) off into an interior subnet and put it behind an 
> actual Linux box?
> 
> Come to look at it, I guess it would have to be the lower half, or I'd 
> have to re-number my router. Not the end of the world, but no sense 
> adding pointless work, either.
> 
> I know I lose three addresses in the process for the new network, 
> router, and broadcast address, but is it *possible*? Does it work? If 
> so, could I get a hand setting it up?
> 

The important thing is that every device knows where to send the 
packets. This happens on both the Ethernet and IPv4 level. IPv6 too if 
you use it.

Subnetting such small networks can waste a lot of addresses because 
making a subnet creates a network address, a gateway address and a 
broadcast address.

An alternative is to use host routes or proxy ARP. With host routes 
there's no subnet. It simply describes that this one IP address is on 
this network, or to use a particular gateway to get to it.

With Proxy ARP the gateway device for your "sub-sub-network" claims to 
own all of the IP addresses behind it. That way, all Ethernet packets 
for those IP addresses go to the gateway, which forwards them to its 
connected networks. The devices on those networks send their packets 
back using the default gateway.

Or kind of like doing Proxy ARP in both directions, you set your 
interior "router" to act as a switch, or bridge, and it forwards IP 
packets in all directions by tracking and forwarding based on detected 
MAC addresses.

Those alternatives can save a lot of IP addresses when working with 
small IPv4 networks.


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