Anyone know why Comcast assigns a weird subnet mask?

[MarcoS]

Just switched ISPs and discovered Comcast assigned a subnet mask on my WAN connection of 255.255.254.0 (my LAN subnet mask is 255.255.255.0). I've read through various how-to's on the topic but I still don't understand the practical difference between the two masks.

Is Comcast giving customers the ability to run two subnets? Our Brother 5250DN picked up the 254 subnet mask and appears to run fine. Should we bother to try and change it back to our LAN mask of 255?

[schooltech]

255.255.255.0 gives you 256 host addresses, but only 254 usable host addresses.

/24 CIDR

0 & 255 are reserved for the network address and the broadcast address, and cannot be assigned to a host, such as a computer, printer, scanner, etc.

255.255.254.0 This gives you 512 host addresses, with 510 usable.

/23 CIDR

If you need, for instance 255 hosts, then you have to go up to the next address, done in binary, to accomplish that.

Because the .255 octet only gives you 254 usable hosts on the subnet, then you have to have 9 host bits to get the 255 minimum, but now up to 510 total usable hosts on the .254 subnet.

So if they need, for instance, 'space' for between 255 and 510 hosts on one network, like a neighborhood, then they will put them on that subnet. This is my understanding of how cable modems work in a neighborhood situation.

I know there is a ton of stuff on the net of how cable modem networks are set up. I've never really looked at it much-mine just works.

Most larger business use a non-routable IP range, such as 10.x.x.x, because it is a class A address range, with a 255.0.0.0 default subnet mask, which would give them 16 million + usable hosts.

This is scalable for most all businesses.

On a sidenote, the current IPv4 gives 4 billion usable, routable IP addresses, which is why they will eventually be tapped, and why subnetting and now supernetting and NAT is beneficial to businesses.

IPv6, I believe, will give each person on earth something like 4 billion addresses each, because it is 128 bits long, rather than the current 32 bit IPv4 address.

Whew, I had some flashbacks of comp. sci. classes in college on this one.

What sucks is that if you don't use this stuff, you quickly forget it.

If I've missed or overlooked something, please chime in (it's been a while.)

[10871087]

[quote=schooltech;38712]What sucks is that if you don't use this stuff, you quickly forget it.[quote]

Rember SUpernetting for the first time, it still makes me want to drink.

Here is a link to a cool subnet calculator, you can plug in you numbers an see how it changes your ranges and other cool stuff.

http://www.subnet-calculator.com/

It's Comcastic!

[unisys12]

Funny this subject comes up! Since I am , litely studying for my Net+, I asked an IT guy about this the other day. I set up two machines for him and noticed that in one building the subnet was 255.255.255.0 and in the next building it was 255.255.254.0, so I had to ask. I explained that I had a basic concept of subnetting. He had a hard time explaining it to me, but managed to get the concept across in a way that I could understand it.

After I got home, I started looking through my Net+ study guide and found there was no mention of supernetting, so... simple subnetting yes, but nothing about supernetting. Thank God! Only problem one problem though, I possibly forgot half the material for the A+, so I have to hit the practice tests again before I go take it.

[RicohTech_TurksCaicos]

It's funny because someone nailed it on the head. If you don't use this knowledge you tend to forget. I have multiple certs A+,Net+,Security+,Server+,HP APS for proliant servers,HP APS for commerical desktops,laptops, and workstations. Among other certs but sometimes you just simply forget things. Now taking CCNA class we are back to basics LOL OSI Layers,converting Hex numbers, etc. Alot of jargon but in pratical terms when you learn the stuff it comes second nature withought the geekness terms of explanation.

I do believe we are somewhere around less that 1million IP4 addresses available IP6 needs the turn on switch very soon.

* from my understanding comptia has updated some of there courses to reflect current tech & different OS technologies.

* Comcast was just recently on the news during a FCC hearing regarding bandwith throttling customers who use p2p and Bitorrents. Comcastic sucks, Optonline better, and if you are in market that offers FIOS thats even better. Heck they even offer Symetrical 20/20 and in some markets 60/60 with promises of more bandwith by years end. But if Cable Isp providers manage to roll out DOCSIS 3 then you can see speed increases double if not triple but my bet is on FIOS.