Question about Subnets

MenuSourceAuto

Hi all,

Taking an interest in networking over the past few weeks and have been learning a bit about subnetting most recently. All going okay so far barring one point regarding how subnets are used from an ISP perspective.

No tutorials or guides mention ISPs in their discussions (that I have seen), and seem to be referring to large corporate networks presumably when explaining the concepts.

My question really is: say my public IP and Subnet Mask as provided by my ISP is 192.168.1.10 and 255.255.255.0, does this mean that the ISP's Network Address is 192.168.1.0, and any router (customer) to which the same ISP assigns an IP address would have an IP of 192.168.1.x. In this case, my ISP provided router is the Host?

And thus they can have max 255 (253?) customers?

I'm still learning the basics but this is something that's been puzzling me and I don't anticipate any of the resources I've been checking to address this point specifically!

Thanks for your time

bonzodog2

192.168.1.10 and 255.255.255.0 are provided by your router, they are not pulic routable IPs. They're translated by the router using NAT

MenuSourceAuto

bonzodog2 wrote: »

192.168.1.10 and 255.255.255.0 are provided by your router, they are not pulic routable IPs. They're translated by the router using NAT

Hi Bonzodog2,

yeahh I understand that - was speaking hypothetically. maybe using those examples were poor but I wouldn't be able to construct a more complex and valid real life example! Can you still see the point I'm trying to make?

Kensington

No, your router has two sides to it - internal and external.

Think of it as your iPad is a device on a network in your house.
Your router is in turn a device on a network in your ISP.

Your ISP might assign you an IP address in the 172.16.1.0 network with mask 255.255.255.0 on the external side.
It might have gateway 172.16.1.1 which is the router for the ISP network.
It then issues customers, including you, with an IP address in the range 172.16.1.2 through 172.16.1.254.
(Note 172.16.1.0 is a private IP but I obviously don't want to use public ranges)

Your router then might have a 192.168.1.0 network with mask 255.255.255.0 on it's internal side.
It might have gateway 192.168.1.1 which is the router for your network.
It then issues your devices with an IP address in the range 192.168.1.2 through 192.168.1.254.

Your router uses NAT to translate from the ISP IP on the external side to the devices on the internal side.


Things get even more complex in the business world of ISP.

Re-using the above example, an ISP might provide you with a static IP in a network 192.168.1.0 with a mask of 255.255.255.252.
The gateway IP 192.168.1.1 is the ISP router for you to use.
Your static IP is 192.168.1.2 is your router device.
The IP 192.168.1.3 is the broadcast address.

The guy down the road gets a static IP in a network 192.168.1.4.
The gateway IP 192.168.1.5 is the ISP router for him to use.
His static IP is 192.168.1.6 is his router device.
The IP 192.168.1.7 is the broadcast address.

Cuddlesworth

MenuSourceAuto wrote: »

My question really is: say my public IP and Subnet Mask as provided by my ISP is 192.168.1.10 and 255.255.255.0, does this mean that the ISP's Network Address is 192.168.1.0, and any router (customer) to which the same ISP assigns an IP address would have an IP of 192.168.1.x. In this case, my ISP provided router is the Host?

And thus they can have max 255 (253?) customers?

On a high level pretty much.

Every network that is defined by a subnet has to have at least 1 gateway device to get out. Traditionally this device is assigned to the first IP in the subnet.

ED E

RAS pools with /32 assignments kinda break the subnet concept, which would be the majority of the connections in the country up until about a year or two ago.

ISP pools dont need to really act like proper "netoworks" as for the most part all traffic is out to the edge routers to get to content and not peers talking to each other.