Subnetting

w^rl0rdw^rl0rd Senior MemberMember Posts: 329
O.K.

Lets say I have an IP of xxx.xxx.074.002
and a subnet of 255.255.255.248

My Net ID is xxx.xxx.074.000 right?

Well if my studies serve me correctly, with that subnet, my Net ID should start at xxx.xxx.074.008. But I'm pulling an xxx.xxx.074.000 w/ my Default Gateway of .001 and IP at .002. What gives?

Comments

  • w^rl0rdw^rl0rd Senior Member Member Posts: 329
    Anybody?
    Maybe I should have posted this in the CCNA forum.

    The reason I'm asking is because this is contrary to what Meyers' Net+ book teaches.
  • bellboybellboy Senior Member Member Posts: 1,017
    it is more like ccna. all you need for n+ with regard to ip addressing is the default subnet for each class.
    A+ Moderator
  • 2lazybutsmart2lazybutsmart Senior Member Member Posts: 1,119
    First, you should write the IP address without the trailing zero's. i.e. x.x.74.2/29. Now the /29 is just the same as writing 255.255.255.248.

    Now if I assume your network is a class C network with a subnet mask of /29, your first subnet ID would be:

    256-248 = 8 and you're first host ID would be 9. You can't use the 0 subnet ID. So that means you can't use host ID's 1-7. The broadcast address for your "8" subnet would be 15. This is becuase the broadcast address is the number right before the next subnet ID.

    Thus, the IP address 192.168.74.9's network ID would be 192.168.74.8

    Hope that helps.
    2lbs.
    Exquisite as a lily, illustrious as a full moon,
    Magnanimous as the ocean, persistent as time.
  • w^rl0rdw^rl0rd Senior Member Member Posts: 329
    Thanks 2lazy. That is what I've learned.

    However, My default gateway is .1 and my IP is .2, even w/ my subnet at /29. My Net ID isn't .8 like it should be. However, I read that theoretically this is OK, as long as my Host ID is not .8, .16, etc. It even says this in the Meyer's book in the note on p320.

    By the way, my ISP set this up, not me. I'm sure I could telnet into my router and fix it if I was real **** about it. This is just the first time I've seen the "rules" broken. Thanks again.
  • 2lazybutsmart2lazybutsmart Senior Member Member Posts: 1,119
    I'm not "100%" sure why we can't use subnet "zero" either. I just follow what some guy called Todd Lammle says (lool). But he doesn't explain why we can't use subnet zero when you have a class C address (although I've seen other people say something about that). However, he does explain the use of subnet 0 in class B and A. The key to both are you shouldn't use subnet 0 if the bits in the third octect are all off (i.e. 0's). and you can't use mask ID (like 192, 224, etc.) if the bits in the third octect are all on (i.e 1's)

    So your configuration would also depend on whether you're using a Class B, or class A network.

    But with what I've learned, you shouldn't use subnet 0 in a class C address while you're around "Cisco". icon_wink.gif

    2lbs.
    Exquisite as a lily, illustrious as a full moon,
    Magnanimous as the ocean, persistent as time.
  • mwgoodmwgood Senior Member Member Posts: 293
    Hi,

    I'm curious where you've learned not to use the first subnet available? In this case, the block from x.x.x.0 - x.x.x.7 ? Does it really say not to do so in the Lammle book? If so, where?

    If your IP address is x.x.x.2 with subnet of 255.255.255.248 then your subnet MUST be x.x.x.0 - x.x.x.7 with network address of x.x.x.0 and broadcast address of x.x.x.7 and usable range of .1 (gateway IP) to .6.

    -Mike
  • w^rl0rdw^rl0rd Senior Member Member Posts: 329
    This is why....

    Net ID: 192.168.074.000
    Sub Mask:255.255.255.248

    .248 = 11111000 (Add 5 Net ID bits)

    2^5=32-2=30 subnets

    Subnets:
    ========
    00000 = n/a
    00001 = .8 <
    ...
    11111 = n/a

    It is not that it won't work, but it is unneccessary to subnet.
    I could get a .1 w/ a standard class C subnet.
  • mwgoodmwgood Senior Member Member Posts: 293
    The method you use for the number of subnets is correct, but isn't really important in this example.

    I don't know if this helps, but to get the "block size" I just subtract the 248 from 256 which gives 8.

    So, knowing the block size of eight, starting with zero - gives you the first block of .0 to .7 with .0 being the network address and .7 being the broadcast (both non-usable).

    Yes, you could use the same IP without subnetting, by using a class C for example. But, that doesn't make subnetting unnecessary - since you can still divide up IPs into subnets this way and make multiple networks.

    -Mike
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