What is the 3rd group of #'s in an IP address and why do they gotta be the same

Do each of the 4 groups of numbers in an ip address have their own name?
Particularly the 3rd group.

If computers on a network dont share the same 3rd group number they wont see
each other on a network, or access the internet if that number doesn't match
up with the one your router is on. I'm just wondering about this

Robert Blackwell

"Robert Blackwell" <> wrote in message
news:...
> Do each of the 4 groups of numbers in an ip address have their own name?
> Particularly the 3rd group.
>
> If computers on a network dont share the same 3rd group number they wont
see
> each other on a network, or access the internet if that number doesn't
match
> up with the one your router is on. I'm just wondering about this
>
>
not that i have heard of. and there is nothing magic about the 3rd octet in
the address. the key is the 'subnet mask'. all of the bits included in the
subnet mask have to match for two devices to talk directly to each other.
normally the subnet mask defaults to 255.255.255.0 which means that the
first 3 octets have to match for the two devices to talk. but if the
subnet mask is 255.255.0.0 then only the first 2 have to match.

so if the subnet mask is 255.255.255.0 the following groups can all talk to
each other:
192.168.0.0 through 192.168.0.255
192.168.1.0 through 192.168.1.255
10.0.0.0 through 10.0.0.255
but if the mask is 255.255.0.0 then the following ranges work:
192.168.0.0 through 192.168.255.255
10.0.0.0 through 10.0.255.255

you have to think binary for finer breakdowns... a mask like 255.255.255.240
gives ranges like:
192.168.0.0 through 192.168.0.15
or a mask like 255.255.240.0 gives ranges like:
192.168.0.0 through 192.168.15.255

Dave

Okay, that makes sense and helps a lot.

See, I just added wifi to my current network but the routers ip is like
192.168.2.1 and it would dhcp any wifi defices to 2.x so the hard wired
computers couldn't see em, I had to manually set em to a static ip of .0.x

So, now it's my understanding that if I configthe lan ip of the wifi router
to a subnet of 255.255.0.0 any wifi devices it dhcp's will still be able to
access the main network, but not the otherway around?

Most impotant for me is just being able to have the wifi devices access
resources on the network which is why I had to static them to 0.x
But our primary router doesn't show the wifi router as an "attached device"
so I can't configure further it either without changing my ip.

Robert Blackwell

"Robert Blackwell" <> wrote in message
news:...
> Okay, that makes sense and helps a lot.
>
> See, I just added wifi to my current network but the routers ip is like
> 192.168.2.1 and it would dhcp any wifi defices to 2.x so the hard wired
> computers couldn't see em, I had to manually set em to a static ip of .0.x
>
> So, now it's my understanding that if I configthe lan ip of the wifi
router
> to a subnet of 255.255.0.0 any wifi devices it dhcp's will still be able
to
> access the main network, but not the otherway around?
>
> Most impotant for me is just being able to have the wifi devices access
> resources on the network which is why I had to static them to 0.x
> But our primary router doesn't show the wifi router as an "attached
device"
> so I can't configure further it either without changing my ip.

you probably have a conflict with two dhcp servers on the network now. you
don't want to mix subnet masks, that won't work either. with what you
describe the wireless devices will be able to send to the lan devices, but
the lan devices will try to reply through their router as if it was an
outside address which apparently your routher can't handle. the best way to
fix it is to disable the wireless router's dhcp server service and let the
lan router handle all the ip's so they are all on the same subnet.
alternately you could play with manually configuring routing tables and
manually assigning ip's, masks, gateways, etc... but i wouldn't want to try
it. i just had a friend go through a similar mixup and it can be a
confusing situation to try to sort out unless you are real familiar with
tcp/ip addressing and all the device configurations

Dave

Another way to think about it is that any traffic directed anywhere other
than the current subnet will be sent to and through the WAN port of the
router/gateway. Thus, if you us a subnet mask of 255.255.255.0, your IP
address is 192.168.1.100 and you are trying to reach 192.168.2.100, this
will be sent to the router/gateway and then through the WAN port of the
router/gateway. On the other hand, if your network (computer and router) is
configured to use a subnet mask of 255.255.0.0 then this will be sent
through the LAN port(s) of the router (not the WAN port).

-Yves

"Robert Blackwell" <> wrote in message
news:...
> Okay, that makes sense and helps a lot.
>
> See, I just added wifi to my current network but the routers ip is like
> 192.168.2.1 and it would dhcp any wifi defices to 2.x so the hard wired
> computers couldn't see em, I had to manually set em to a static ip of .0.x
>
> So, now it's my understanding that if I configthe lan ip of the wifi
> router to a subnet of 255.255.0.0 any wifi devices it dhcp's will still be
> able to access the main network, but not the otherway around?
>
> Most impotant for me is just being able to have the wifi devices access
> resources on the network which is why I had to static them to 0.x
> But our primary router doesn't show the wifi router as an "attached
> device" so I can't configure further it either without changing my ip.
>
>
>

Yves Konigshofer