WAN/LAN IP Address

Discussion in 'Cisco' started by AlanNg, Dec 9, 2004.

  1. AlanNg

    AlanNg Guest

    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 !
    AlanNg, Dec 9, 2004
    #1
    1. Advertising

  2. AlanNg

    Enno Lenze Guest

    AlanNg wrote:
    > 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

    --
    http://www.verbrennung.org
    Enno Lenze, Dec 9, 2004
    #2
    1. Advertising

  3. AlanNg

    Toby Guest

    "AlanNg" <> wrote in message
    news:cp9ccn$a6m$...
    > 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
    Toby, Dec 9, 2004
    #3
  4. AlanNg

    Scooby Guest

    "Enno Lenze" <> wrote in message
    news:...
    > AlanNg wrote:
    > > 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.


    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.

    >
    > > - 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)


    That says it.

    >
    > > - 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


    Layer 1

    >
    > > - 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.
    >


    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.

    > > 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
    >
    > --
    > http://www.verbrennung.org


    Jim
    Scooby, Dec 9, 2004
    #4
  5. AlanNg

    CZ Guest

    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 ?!
    CZ, Dec 10, 2004
    #5
  6. AlanNg

    AlanNg Guest

    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" <> wrote in message
    news:...
    > 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, Dec 10, 2004
    #6
  7. AlanNg

    CZ Guest

    >> 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.
    CZ, Dec 10, 2004
    #7
  8. AlanNg

    AlanNg Guest

    CZ, understand, thnks !

    "CZ" <> wrote in message
    news:%...
    > >> 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, Dec 12, 2004
    #8
    1. Advertising

Want to reply to this thread or ask your own question?

It takes just 2 minutes to sign up (and it's free!). Just click the sign up button to choose a username and then you can ask your own questions on the forum.
Similar Threads
  1. kev
    Replies:
    4
    Views:
    462
    Scooby
    Nov 17, 2003
  2. AM
    Replies:
    1
    Views:
    545
    Phillip Remaker
    May 24, 2005
  3. Euclides
    Replies:
    6
    Views:
    2,574
    Euclides
    Jan 26, 2006
  4. thrill5
    Replies:
    3
    Views:
    1,101
    stephen
    Jul 22, 2006
  5. Replies:
    1
    Views:
    568
    stephen
    Jul 26, 2006
Loading...

Share This Page