Who knows about subnet masks?

[mkatts]

And subnetting down to a 30 bit network does not prevent you from scanning any other network. You can scan any IP range in the world. All the subnet mask does is define the size of the local network so the device knows to either talk to a device as a local network device using each others MAC address or to send the packet to the configured default gateway.

I will use a /29 subnet as an example instead.
say you have a network of 192.168.0.0/29
192.168.0.0 = NeworkID
192.168.0.1 = usable
192.168.0.2 = usable
192.168.0.3 = usable
192.168.0.4 = usable
192.168.0.5 = usable
192.168.0.6 = usable
192.168.0.7 = broadcast

so lets say that .1 is the default gateway or default route.
You have your laptop using .2, you desktop is .3 and your printer is .4.
You want to connect from your laptop to your desktop. When .2 trys to connect to .3 he will look at the destination IP and realize it's in his local network so he will send an ARP (address resolution protocol) request to ge the MAC address of .3. Once he gets that, he will send data with to the MAC address and not the IP address.

Now lets say you want to connect to Satelliteguys.us. Ip address is 24.28.199.152.

So .2 will look at the IP address and understand that it's not local, so he will send the request to his default gateway.

So if you have a /30 home network, it does not prevent you from seeing your neighbors local network. That does mean the provider has not done something else to prevent it, but it not simply because the of the subnet mask.

By the way, I never understood why providers use a /30 for point to point IP'. It's a huge waste. Should always use a /31 bit where you have two IP's in the network. A network ID and a broadcast. Since it's only two devices talking, it does not matter about the network or broadcast designation. Get more bang for you buck when IP's are at a premium.

 

[John Kotches]

No, 128 is the network, 131 is the broadcast address for the network, only 129 and 130 are assignable.

I had the impression this is not true anymore with modern network equipment...

Try it and see how well it works. Is "impression" another way of saying "I've never tried this"?

Neither A "consumer level" nor a "commercial grade" router are allowed to route private IP addresses...

Yes. But some Ciscos can be easily made do that...

Please read RFC1918 from the IETF, then we can discuss this further. You can route these just fine (within an organization / ISP) it's going outside of this controlled environment that you can't.

I deal with this on a daily basis for machines that are publically facing.

Not sure how third party firmware (e.g. DD-WRT, Sveasoft) treat them. Stock Linksys will drop them.

Diogen.

Don't know, haven't had the time or the need to futz with 3rd party firmwares.

 

[diogen]

"impression" another way of saying "I've never tried this"?

I'm not as ISP, so yes, I haven't tried it. And since it is not what I have to deal with on a daily basis, I've neither time nor need to futz with it.
About a year ago I was on a presentation given by a couple Cisco guys talking about their latest products. Can't say I'm sure they
were talking exactly about this, but I got the "impression" this "IP waste" (their term) is not needed anymore.
I didn't know that "impression" is such a bad word in the net pros community, won't use it again.

Please read RFC1918 from the IETF, then we can discuss this further.

I won't read it and we won't discuss it. But thanks for the offer.

And for the lesson.

Diogen.

 

[Smith P.]

Just for those ppl who want to know more - here is Wiki link what will help you calculate number of hosts by using particular subnet mask.