Gateway Address of 10.1.1.255 Valid?

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Practicing for my Intro exam and have run across several questions from the
test exam of the Cisco Press CCNA Intro Exam Certification Guide.

Several diagrams list host configurations with a gateway address of
10.1.1.255 but the router Ethernet port is 10.1.1.254.  A /24 subnet mask is
used - I assume the gateway address with the 255 is the broadcast address?

I've never seen a config set like that.



Re: Gateway Address of 10.1.1.255 Valid?
I am only on Sem3 of CCNA- but I have always thought that 255 is the
broadcast- ?
Cisco Kid wrote:
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Re: Gateway Address of 10.1.1.255 Valid?
/24 mask in 255.255.255.0. So in your case, 10.1.1.1255 should be the
broadcast address, I would guess.


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Re: Gateway Address of 10.1.1.255 Valid?

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That's what I thought, too.  But here it is - the official Cisco Press
640-821 Practice Exam (updated from the web site) and it has at least 4
instances of a diagram listing a single router, several switches and 4
hosts.  One host has a correct GW address of the router's ethernet interface
(10.1.1.254).  The other 3 hosts have a GW address of 10.1.1.255 (which I
assume is incorrect).

The instructions say to keep taking these practice exams until you get 100%
correct - I guess it'll never happen with me.

Unless there's some trick not in the books which allows a broadcast address
to be used as a GW address?

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Re: Gateway Address of 10.1.1.255 Valid?
I will give you the actual answer - no guessing:

In this situation, 10.1.1.255 is the broadcast IP address for that subnet
and cannot be used by any host.

In any subnet, the highest and lowest IP address are not used.
    Usable IP addresses = IP addresses in subnet - 1 IP address for the
Subnet ID - 1 IP address for the Broadcast IP
    254 = 256 - 1 - 1
    254 IP addresses remain and your router will use one of them leaving 253
for other hosts.

Example:
    192.168.1.0 255.255.255.0 (aka 192.168.1.0 / 24)
    Lowest IP    - 192.168.1.0        (subnet ID)
    Highest IP    - 192.168.1.255    (broadcast IP)

Want to really challenge your brain?  Check this out:
    192.168.1.0 255.255.254.0 (aka 192.168.2.0 / 23)
    Lowest IP    - 192.168.2.0        (subnet ID)
    Highest IP    - 192.168.2.255    (broadcast IP)
    In this situation, the IP address 192.168.3.0 is completely valid for a
host!!!  That is because it is not the highest or lowest IP address in the
subnet.

Salutations,
Lord Balki
CCNA, 1/2 CCNP, 12 years in the field



CORRECTION - Re: Gateway Address of 10.1.1.255 Valid?
CORRECTION:

Want to really challenge your brain?  Check this out:
    192.168.2.0 255.255.254.0 (aka 192.168.2.0 / 23)
    Lowest IP    - 192.168.2.0        (subnet ID)
    Highest IP    - 192.168.2.255    (broadcast IP)
In this situation, the IP address 192.168.3.0 is completely valid for a
host!!!  That is because it is not the highest or lowest IP address in the
subnet.



Re: CORRECTION - Re: Gateway Address of 10.1.1.255 Valid?

"Overlord" <Overlord Balki> wrote in message
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That I understand but don't know why anyone in the real world would want to
subnet (or supernet?) a 192.168 scheme with a /23 mask.
I just thought it incredibly sloppy for Cisco Press to issue a diagram with
the .255 gateway address when that was clearly reserved for the subnet
broadcast value.

Guess it doesn't matter - I did manage to pass the Intro exam yesterday...



Re: CORRECTION - Re: Gateway Address of 10.1.1.255 Valid?
Cisco Kid wrote:

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It's private addressing, so one is free to do as one wishes.

While I personally prefer to see subnets of /24 and smaller, it can make sens
to use a /23.

If you have a building, with logical areas (eg floors or buildings within a
campus) that need for example 300 addresses, using a /23 keeps it simple. That
way someone on floor 3 has a problem, you *know* immediately which subnet and
hence VLAN they are in without having to check.

The best way to do things varies with time. It is not too long ago[1] that the
phrase "switch where you can, route where you must" was popular. There used to
be a *significant* performance hit if you needed to go via the router from VLAN
to VLAN, so you will still see some networks with rather large subnets. I
regularly see /20s for users. I know of one large network that has over 2000
users in one VLAN - the "customer" did not understand VLANs and would not let
the network bods implement VLANs. If I also add that many were noebook users,
and portfast was not enabled you can imagine how bad it was.

Bear in mind that an awful lot of what we consider to be good practice is based
upon opinion.

P.

[1] or am I showing my age?
--
Paul Matthews                                         CCIE #4063
Please post questions to the NG, NOT by e-mail.

Re: Gateway Address of 10.1.1.255 Valid?
Cisco Kid wrote:

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You are right, it is a misconfiguration

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There's lots of ways people get things wrong.

remember as well that there is passing the exam, and there is reality.

Theoretically it is possible for the all ones (broadcast) gateway to work. In
reality I would home that if you tried to set a device up address 10.1.1.43,
mask 255.255.255.0 gateway 10.1.1.255 that it should report an error. Reality
says that depends on whoever coded that interface to have thought to check for
correct. Should the end system allow it you then have an interesting
position...

P.
--
Paul Matthews                                         CCIE #4063
Please post questions to the NG, NOT by e-mail.

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