WAN/LAN IP Address

[AlanNg]

Hi, recently my office purchase a Cisco 1721 router and the vendor has
configured the WAN IP as 203.X.X.250/30 and LAN IP as 203.X.X.224/27. The
WAN IP's subnet mask is 255.255.255.252 in subnet address of 203.X.X.248
with possible WAN IP as either 249 or 250 which is correct, ie. 250 used. As
for the LAN IP, with the subnet mask of 255.255.255.224, the subnet address
is 203.X.X.224 with possible LAN IP from 225 to 254, but how come the LAN IP
is configured same as the subnet address,ie. 224, isn't there a conflict ?!

Next, there are some queries that I don't really understand, hoping someone
can enlighten me;
- How is subnet address being used, assign to where/what ?!
- What is the LAN IP for, in the case of router, I thought router should
only have one IP address ?!
- Hub is defined in layer 1 or 2 of OSI ?!
- Can we identify the class of an IP address by the first octet of the
address, ie. 1-126 must be in class A, 128-191 must be in class B and so
on, without the need to pay attention to its subnet mask ?!

Please pardon me if I ask silly questions as I'm rather new to these area,
and hope to rx help asap, thanks !

 

[Enno Lenze]

is configured same as the subnet address,ie. 224, isn't there a conflict ?!

no, it isn't the subnet mask isn't an adress or someting like that.
it's only a "rule" how many adresses you can allocate. Simple reminder:
subtract the subnetmask from 255.255.255.255 and you know how much hosts
you have.

- How is subnet address being used, assign to where/what ?!

it is used in cobination with teh ip-address i.e.
192.168.0.1 subnetmask 255.255.255.0 means:
in the first three blocks you can cahnge 0 things (remember my lines
above: 255-255=0, you have 0 other possibilities)
the last block in the subntmask is 0 (as above 255-0=255, so you have
255 possibilties in the last block).

means:
you can allocate 192.168.0.1 - 192.168.0.255

(hope you unterstand what i want to say, yo tired to explain it better)

- What is the LAN IP for, in the case of router, I thought router should
only have one IP address ?!

no, the router needs one adress in each network, because the different
networks cant communicate. i.e.:
you have two school classes. both can see the teacher. each class can
see the member of its class, but not the other. the teacher is the
router, because he's visible for both, he has to give the informations
from one room (subnet) to the other

- Hub is defined in layer 1 or 2 of OSI ?!
dunno

- Can we identify the class of an IP address by the first octet of the
address, ie. 1-126 must be in class A, 128-191 must be in class B and so
on, without the need to pay attention to its subnet mask ?!

well..yes and no
long long time ago that was the plan, but today you have to use the
subnetmask to know your network size.

Please pardon me if I ask silly questions as I'm rather new to these area,
and hope to rx help asap, thanks !

no question is silly, communication is what usenet is about

regards, enno

 

[Toby]

Hi, recently my office purchase a Cisco 1721 router and the vendor has
configured the WAN IP as 203.X.X.250/30 and LAN IP as 203.X.X.224/27. The
WAN IP's subnet mask is 255.255.255.252 in subnet address of 203.X.X.248
with possible WAN IP as either 249 or 250 which is correct, ie. 250 used.
As
for the LAN IP, with the subnet mask of 255.255.255.224, the subnet
address
is 203.X.X.224 with possible LAN IP from 225 to 254, but how come the LAN
IP
is configured same as the subnet address,ie. 224, isn't there a conflict
?!

Next, there are some queries that I don't really understand, hoping
someone
can enlighten me;
- How is subnet address being used, assign to where/what ?!
- What is the LAN IP for, in the case of router, I thought router should
only have one IP address ?!
- Hub is defined in layer 1 or 2 of OSI ?!
- Can we identify the class of an IP address by the first octet of the
address, ie. 1-126 must be in class A, 128-191 must be in class B and so
on, without the need to pay attention to its subnet mask ?!

Please pardon me if I ask silly questions as I'm rather new to these area,
and hope to rx help asap, thanks !

I'm not sure how your ISP has achieved this as although theoretically
possible as long as you dont allocate IP addresses on your LAN in the range
203.x.x.248-251 it is not a clever setup, I am not sure of the IOS you have
but if I try this on my 2501 in my lab running I get the following error
messages.

(config-if)#ip address 192.168.0.250 255.255.255.252
192.168.0.248 overlaps with Ethernet0

Ethernet0 is configured as 192.168.0.11 255.255.255.0 but the config allows
the setting but with warning.

(config-if)#no shut
192.168.0.248 overlaps with Ethernet0
Serial0: incorrect IP address assignment

And the Interface stays administratively down.

I would be intrested if anyone else has seen this type of behavour from an
ISP before as if possible could be a problem waiting to happen.

regards

Toby

 

[Scooby]

?!

no, it isn't the subnet mask isn't an adress or someting like that.
it's only a "rule" how many adresses you can allocate. Simple reminder:
subtract the subnetmask from 255.255.255.255 and you know how much hosts
you have.

Yes, there is a conflict. This should NEVER be done. I'm usually a fan of
never say never, but don't do this. In fact, I'm not even sure how they
were able to configure the equipment as such, since the IOS barks out errors
when you try to put two interfaces on the same network. The ONLY exception
to this rule would be if they were bridging the interfaces together, but if
they were doing that, the subnet masks should match as well. This is just a
bad setup.

it is used in cobination with teh ip-address i.e.
192.168.0.1 subnetmask 255.255.255.0 means:
in the first three blocks you can cahnge 0 things (remember my lines
above: 255-255=0, you have 0 other possibilities)
the last block in the subntmask is 0 (as above 255-0=255, so you have
255 possibilties in the last block).

means:
you can allocate 192.168.0.1 - 192.168.0.255

(hope you unterstand what i want to say, yo tired to explain it better)

That says it.

no, the router needs one adress in each network, because the different
networks cant communicate. i.e.:
you have two school classes. both can see the teacher. each class can
see the member of its class, but not the other. the teacher is the
router, because he's visible for both, he has to give the informations
from one room (subnet) to the other


dunno

Layer 1

well..yes and no
long long time ago that was the plan, but today you have to use the
subnetmask to know your network size.

You've always had to use the subnet mask in conjunction with the ip address
(maybe in some cases it was not visible to the person configuring the
equipment), and you've always been able to subnet. The classes still exist
today as well. What CIDR and VLSM introduced is that you can now subnet
beyond class boundaries. There is still equipment out there there requires
a n*8 bit subnet mask, but now most equipment can have any length subnet
mask.

no question is silly, communication is what usenet is about

regards, enno

Jim

 

[CZ]

Re: routers, IP addresses, and misc comments

Alan:

Your router's WAN port address is set as 203.X.X.250/30 (/30 =
255.255.255.252 subnet mask). That subnet allows two addresses to be used
for hosts as 203.x.x.249 & 203.x.x.250, and 203.x.x.248 as the network
address, and 203.x.x.251 as the broadcast address.

Your router's LAN port address is set as 203.X.X.224/27 (/27=
255.255.255.224 subnet mask), which is not correct. That subnet allows 30
addresses to be used for hosts, from 203.x.x.225 thru 203.x.x.254, and
203.x.x.224 as the network address, and 203.x.x.255 as the broadcast
address. You should change the LAN port's address accordingly (i.e., use
..225 instead of .224). BTW, review this with your vendor.

Re: a host's address and its subnet mask having the same 4th octet value
(i.e., 224):
Generally, a host should not have the same decimal value for the 4th octet
as the same octet in the subnet mask, as that value would be used for the
network address in those situations where this identical value existence
occurs.

Re: IMO, general subnetting theory:
Note1: an octet has 8 bits, and a bit has a binary value of either a 0 or a
1.
Note2: subnet mask octets only have contiguous binary 1s from the left, or
all 0s.
An IP address has two basic components: network bits (Class octets), and
host bits (non-Class octets).
Subnetting is the division of the host bits into subnet IDs, and host IDs
per subnet ID.
The subnet mask is used to indicate if subnetting exists, and if it does
exist, what the addresses for the subnet are.
If the subnet mask has binary 1s in a corresponding host bit octet, then
subnetting exists, and the positions of those subnet mask binary 1s compared
to the binary value of the corresponding IP address octet defines the
subnet's addresses.
Generally, the first address within the subnet's range of addresses is used
for the network address, and the last address is used for the broadcast
address. The remaining addresses can be used for nodes/devices including a
router's port.

Re: a router's addresses:
The router's LAN port IP address is normally used as the default gateway for
your LAN computers.

Re: OSI layer devices:
OSI layer 1: hubs
OSI layer 2: switches, bridges
OSI layer 3: routers

Re: IP addresses, Class codes, and subnet masks:
The decimal value of the first octet of an IP address indicates the Class
code of the address. The Class code indicates which octets contain the
network bits. The subnet mask indicates if subnetting exists, and if it
does, it also indicates the division of the host bits into a subnet ID, and
the host IDs within that subnet.
Basically, the subnet mask is used to define subnets, and if you have two
subnets, then a router is used to communicate between them (a hub, bridge,
or switch cannot do it).

OP's post:
Hi, recently my office purchase a Cisco 1721 router and the vendor has
configured the WAN IP as 203.X.X.250/30 and LAN IP as 203.X.X.224/27. The
WAN IP's subnet mask is 255.255.255.252 in subnet address of 203.X.X.248
with possible WAN IP as either 249 or 250 which is correct, i.e.. 250 used.
As
for the LAN IP, with the subnet mask of 255.255.255.224, the subnet address
is 203.X.X.224 with possible LAN IP from 225 to 254, but how come the LAN IP
is configured same as the subnet address,i.e.. 224, isn't there a conflict
?!

Next, there are some queries that I don't really understand, hoping someone
can enlighten me;
- How is subnet address being used, assign to where/what ?!
- What is the LAN IP for, in the case of router, I thought router should
only have one IP address ?!
- Hub is defined in layer 1 or 2 of OSI ?!
- Can we identify the class of an IP address by the first octet of the
address, i.e.. 1-126 must be in class A, 128-191 must be in class B and so
on, without the need to pay attention to its subnet mask ?!

 

[AlanNg]

CZ, thanks, you have explained in details, but as for the LAN IP, because
the last octet of the subnet mask is 224, thus we shouldn't use subnet id
203.X.X.224, the last possible subnet id can be use is 203.X.X.192, with
host from 193 to 222 and its broadcast as 223. I think I have made a mistake
in my first mail saying that the LAN IP's subnet id should be 203.X.X.224,
please correct me if I'm wrong, thks again !

 

[CZ]

as for the LAN IP, because
the last octet of the subnet mask is 224, thus we shouldn't use subnet id
203.X.X.224, the last possible subnet id can be use is 203.X.X.192, with
host from 193 to 222 and its broadcast as 223. I think I have made a mistake
in my first mail saying that the LAN IP's subnet id should be 203.X.X.224,
please correct me if I'm wrong

Alan:

If you are asking what subnet mask is valid if the router's LAN port has
203.x.x.224 as its address, the answer is 255.255.255.192 (=/26).
For 203.x.x.224/26, the subnet ID is 203.x.x.192, the broadcast address is
203.x.x.255, and the usable host address range is 203.x.x193 thru
203.x.x.254.

Remember that the subnet ID is the first address in the full range of
addresses of the subnet.

 

[AlanNg]

CZ, understand, thnks !

the last octet of the subnet mask is 224, thus we shouldn't use subnet id
203.X.X.224, the last possible subnet id can be use is 203.X.X.192, with
host from 193 to 222 and its broadcast as 223. I think I have made a mistake
in my first mail saying that the LAN IP's subnet id should be 203.X.X.224,
please correct me if I'm wrong

Alan:

If you are asking what subnet mask is valid if the router's LAN port has
203.x.x.224 as its address, the answer is 255.255.255.192 (=/26).
For 203.x.x.224/26, the subnet ID is 203.x.x.192, the broadcast address is
203.x.x.255, and the usable host address range is 203.x.x193 thru
203.x.x.254.

Remember that the subnet ID is the first address in the full range of
addresses of the subnet.