HELP !...finding a subnet broadcast address....

RyanCorbettRyanCorbett MemberMember Posts: 54 ■■■□□□□□□□
either wendell odom has lost his grapes or i just need a break...either way, i'm pissed because i've hit a wall when everything regarding subnetting was going so well ( although, it has not been the easiest thing to learn )...odom is talking about " drawing vertical lines through rightmost binary 1s/counting up 0s "...i'm fed UP !!...point blank; how does one find the subnet broadcast address when the subnet number and IP are known ?....a simple Boolean AND will not suffice; something tricky has to be done at the end that i can't quite wrap my brain around ( too much coffee today, perhaps ?....i dunno )...odom simply has trouble explaining it....i'd appreciate any help from any CCENTs/CCNAs out there...thanks, guys !....icon_cry.gif

Comments

  • pham0329pham0329 Senior Member Member Posts: 556
    If you know the subnet number, I'm assuming you also know how to find the range based on the mask? If so, the broadcast address is the last ip in the range, or the address where all the host bits are 1s
  • RyanCorbettRyanCorbett Member Member Posts: 54 ■■■□□□□□□□
    pham0329 wrote: »
    If you know the subnet number, I'm assuming you also know how to find the range based on the mask? If so, the broadcast address is the last ip in the range, or the address where all the host bits are 1s

    maybe i did at one point, but have since forgotten how to do it....thanks, though !
  • techie2012techie2012 Network Engineer Member Posts: 150
    either wendell odom has lost his grapes or i just need a break...either way, i'm pissed because i've hit a wall when everything regarding subnetting was going so well ( although, it has not been the easiest thing to learn )...odom is talking about " drawing vertical lines through rightmost binary 1s/counting up 0s "...i'm fed UP !!...point blank; how does one find the subnet broadcast address when the subnet number and IP are known ?....a simple Boolean AND will not suffice; something tricky has to be done at the end that i can't quite wrap my brain around ( too much coffee today, perhaps ?....i dunno )...odom simply has trouble explaining it....i'd appreciate any help from any CCENTs/CCNAs out there...thanks, guys !....icon_cry.gif

    Its easy once you get the concept. Use block sizes for example we know that an ip address is 32 bits separated into 4 octets in each octet you can have a series of 1s and 0s signifying the network and host portion of the Ip. So say we have 11111111.11111111.11111111.00000000 this is a /24 since 24 bits are on for the network and 8 bits are off for the hosts. We know that the number of hosts we can have is 2^n-2 where n= # of 0s and in this case the equation equals 254. The reason we subtract 2 is for the network address and the broadcast address. So if it were 192.168.1.0/24 the hosts would be 192.168.1.1-192.168.1.254 and 192.168.1.255 is the broadcast. The broadcast address is always the last ip before the next network address. For an example with subnetting, the easiest thing to do is to memorize what block size you have for each bit turned on in the octet. When doing this have 128 64 32 16 8 4 2 1 match up with the octet. If you have 11111111.11111111.11111111.100000000 it is a /25 and the block size is 128. Subtract 2 for the network address and broadcast and if we were using 192.168.1.0/25 the hosts would be 192.168.1.1-192.168.1.126 and the broadcast would be 192.168.1.127 and the next network would be 192.168.1.128/25 and the hosts on that would be 192.168.1.129-192.168.1.254 and 192.168.1.255 would be the broadcast. Now if you have a /26 then there are 2 bits on, so you can see that the last turned on bit is under the 64 so you would have a network of .0 and hosts 1-62 and .63 would be broadcast. There is a little more to it than I want to type lol but that should get you started in Class C subnetting and broadcast addresses. There is a link to subnetting material in teh technotes in the CCNA section of the website though. That explain everything. Best of luck!!icon_thumright.gif
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  • xirtlookxirtlook Senior Member Member Posts: 124
    -[7]-[6]-[5]-[4]-[3]-[2]-[1]-[0]-
    .128..64..32..16...8....4....2...1........ (2^n = block size) (ex: 2^2 = 4)
    /01 /02 /03 /04 /05 /06 /07 /08........(so CIDR /6, /14, /22, and /30 are increments of 4bits)
    /09 /10 /11 /12 /13 /14 /15 /16
    /17 /18 /19 /20 /21 /22 /23 /24
    /25 /26 /27 /28 /29 /30 /31 /32

    so if you're in the 3rd octet, and using /30... you are using a blocksize of 4bits.
    so you increment by 4.

    for example: 192.168.10.0/30 => 192.168.10.0,192.168.10.4, 192.168.10.8, 192.168.10.12
    So these are the NETWORK ADDRESSES, and we know the broadcast is the last number BEFORE the next NA.
    so the BA for each subnet would be 192.168.10.3, 192.168.10.7, 192.168.10.11, 192.168.10.15.

    Lets do another one, in the 3rd octet, maybe a /23 CIDR.

    CIDR /23 is 2bits block size, so each sub/network address will be in increments of 2.

    172.16.0.0, 172.16.2.0, 172.16.4.0, 172.16.6.0, 172.16.8.0, and so on.
    remember these are the network addresses, and the broadcast address is just the last ip address within that network before moving onto the next network.

    so the last ip address in the 172.16.0.0 before you get to 172.16.2.0 would be 172.16.1.255.

    I cut alot of corners with charts, but if you need it, i can break this down to binary too.
    just difficult to explain in a thread/forum.
    nerd power.
  • RyanCorbettRyanCorbett Member Member Posts: 54 ■■■□□□□□□□
    techie2012 wrote: »
    Its easy once you get the concept. Use block sizes for example we know that an ip address is 32 bits separated into 4 octets in each octet you can have a series of 1s and 0s signifying the network and host portion of the Ip. So say we have 11111111.11111111.11111111.00000000 this is a /24 since 24 bits are on for the network and 8 bits are off for the hosts. We know that the number of hosts we can have is 2^n-2 where n= # of 0s and in this case the equation equals 254. The reason we subtract 2 is for the network address and the broadcast address. So if it were 192.168.1.0/24 the hosts would be 192.168.1.1-192.168.1.254 and 192.168.1.255 is the broadcast. The broadcast address is always the last ip before the next network address. For an example with subnetting, the easiest thing to do is to memorize what block size you have for each bit turned on in the octet. When doing this have 128 64 32 16 8 4 2 1 match up with the octet. If you have 11111111.11111111.11111111.100000000 it is a /25 and the block size is 128. Subtract 2 for the network address and broadcast and if we were using 192.168.1.0/25 the hosts would be 192.168.1.1-192.168.1.126 and the broadcast would be 192.168.1.127 and the next network would be 192.168.1.128/25 and the hosts on that would be 192.168.1.129-192.168.1.254 and 192.168.1.255 would be the broadcast. Now if you have a /26 then there are 2 bits on, so you can see that the last turned on bit is under the 64 so you would have a network of .0 and hosts 1-62 and .63 would be broadcast. There is a little more to it than I want to type lol but that should get you started in Class C subnetting and broadcast addresses. There is a link to subnetting material in teh technotes in the CCNA section of the website though. That explain everything. Best of luck!!icon_thumright.gif


    this sounds like what i needed...thanks so much !
  • RyanCorbettRyanCorbett Member Member Posts: 54 ■■■□□□□□□□
    xirtlook wrote: »
    -[7]-[6]-[5]-[4]-[3]-[2]-[1]-[0]-
    .128..64..32..16...8....4....2...1........ (2^n = block size) (ex: 2^2 = 4)
    /01 /02 /03 /04 /05 /06 /07 /08........(so CIDR /6, /14, /22, and /30 are increments of 4bits)
    /09 /10 /11 /12 /13 /14 /15 /16
    /17 /18 /19 /20 /21 /22 /23 /24
    /25 /26 /27 /28 /29 /30 /31 /32

    so if you're in the 3rd octet, and using /30... you are using a blocksize of 4bits.
    so you increment by 4.

    for example: 192.168.10.0/30 => 192.168.10.0,192.168.10.4, 192.168.10.8, 192.168.10.12
    So these are the NETWORK ADDRESSES, and we know the broadcast is the last number BEFORE the next NA.
    so the BA for each subnet would be 192.168.10.3, 192.168.10.7, 192.168.10.11, 192.168.10.15.

    Lets do another one, in the 3rd octet, maybe a /23 CIDR.

    CIDR /23 is 2bits block size, so each sub/network address will be in increments of 2.

    172.16.0.0, 172.16.2.0, 172.16.4.0, 172.16.6.0, 172.16.8.0, and so on.
    remember these are the network addresses, and the broadcast address is just the last ip address within that network before moving onto the next network.

    so the last ip address in the 172.16.0.0 before you get to 172.16.2.0 would be 172.16.1.255.

    I cut alot of corners with charts, but if you need it, i can break this down to binary too.
    just difficult to explain in a thread/forum.

    thanks a bunch !
  • RyanCorbettRyanCorbett Member Member Posts: 54 ■■■□□□□□□□
    i was right...too much coffee+exhaustion=no comprehension of a relatively simple concept...i'm able to almost grasp the whole of subnetting now...i'm at the end of wendell odom's subnetting chapter and the last concept is " knowing how to choose a valid, classsful subnet "...doing better guys, thanks !....
  • lsud00dlsud00d 1337sauce Member Posts: 1,571
    I learned subnetting recently...took about 15 minutes and I can do it all in my head now icon_cool.gif

    Should be helpful when going for some networking certs...
  • RyanCorbettRyanCorbett Member Member Posts: 54 ■■■□□□□□□□
    lsud00d wrote: »
    I learned subnetting recently...took about 15 minutes and I can do it all in my head now icon_cool.gif

    Should be helpful when going for some networking certs...

    i hear those with a propensity for math and analytical thinking can learn these things quickly after being in the industry for awhile...for those like me, who were english majors and " math-dumb ", it takes awhile....
  • pham0329pham0329 Senior Member Member Posts: 556
    lsud00d wrote: »
    I learned subnetting recently...took about 15 minutes and I can do it all in my head now icon_cool.gif

    Should be helpful when going for some networking certs...

    Maybe you should share how you learned it in 15 minutes rather than gloating about it to the OP, who's having some difficulties with it
  • RyanCorbettRyanCorbett Member Member Posts: 54 ■■■□□□□□□□
    pham0329 wrote: »
    Maybe you should share how you learned it in 15 minutes rather than gloating about it to the OP, who's having some difficulties with it

    hey pham !...i didn't want to be ugly, but that's kind of how i felt...thanks for being the " voice of the voiceless ! "...anyways, i've got it all working now....thanks for all your input !....:)
  • CodeBloxCodeBlox Network Engineer Member Posts: 1,363 ■■■■□□□□□□
    Ryan,

    I picked up subnetting in a short time simply because I did constant quizzes for a while. Can't say it was in 15 minutes though like the other guy. Glad you found your answer. Also one thing, I notice you are in Hampton, VA. Thats exactly where I live!! Are you attending the netacad program somewhere at a community college locally? I attended the netacad program at TNCC. Great place to learn the material with plenty of hardware
    Currently reading: Network Warrior, Unix Network Programming by Richard Stevens
  • RyanCorbettRyanCorbett Member Member Posts: 54 ■■■□□□□□□□
    CodeBlox wrote: »
    Ryan,

    I picked up subnetting in a short time simply because I did constant quizzes for a while. Can't say it was in 15 minutes though like the other guy. Glad you found your answer. Also one thing, I notice you are in Hampton, VA. Thats exactly where I live!! Are you attending the netacad program somewhere at a community college locally? I attended the netacad program at TNCC. Great place to learn the material with plenty of hardware

    thanks for the reply...i attended the netacademy courses at ECPI about six years ago, and i was a small computer systems specialist for 4 years in the marines ( you'd think i'd have this down by now ! )....yep, i have packet tracer and i'm able to comprehend subnetting completely now, it's just remembering the processes that is proving to be difficult...thanks for the reply !
  • xirtlookxirtlook Senior Member Member Posts: 124
    I went to a bootcamp and watched a video called subnetting made easy.
    picked it up, and practiced it. learned to create my own little chart.
    had it down in 2 days.

    practice it over and over, and you get the hang of it. can subnet, supernet like its nothing.

    memorize multiple s of 16 too. Cisco loves 16.

    16, 32, 48, 64, 80, 96, 112, 128, 144, 160, 176, 192, 208, 224, 240, 256
    nerd power.
  • cb3dwacb3dwa Member Member Posts: 80 ■■□□□□□□□□
    why 16s?

    what advantage does this have?

    dave
  • BrakarificBrakarific Junior Member Member Posts: 23 ■□□□□□□□□□
    I memorized multiples of 8, which is the same concept as 16. If you do that, you should be able to remember a lot of the key numbers for subnetting. xirtlook is right though, Cisco loves multiples of 16 (or 8, same thing) when giving you subnetting scenarios. I can tell you that they really didn't get into anything below /24 when I took the test a year and a half ago. That being said, I have used /22 and /23 subnets at work, so being comfortable with any subnetting scenario is important.
  • cb3dwacb3dwa Member Member Posts: 80 ■■□□□□□□□□
    cheers was thinking of this in the car on the way home from work and realised it was a stupid question lol

    makes so much scene now cheers
  • xirtlookxirtlook Senior Member Member Posts: 124
    cheers mate :D

    sorry I'm american, and when I worked for Under Armour and provided remote support to people in Ireland and Amsterdam, that was the coolest thing to me. Hearing them say "cheers".
    nerd power.
  • djfunzdjfunz Senior Member Member Posts: 307
    If you or anyone ever has issues with subnetting in the future, I would recommend watching the 3 subnetting videos from CBT Nuggets. I was also lost by all the written explanations and after watching his videos, it was so much clearer. It takes a while in the beginning writing everything down, but after a little bit you begin to write less and less to get the problem done. I was at 7-8 minutes in the beginning but now I can knock em out in 2-3 minutes per problem. Jeremy Cioara is the man!
    WGU Progress - B.S. IT - Completed
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