I need help understanding auto summarization

spruggie

I've been trying to understand auto summarization by practicing questions, but i'm making no head way at all. a question I got is, "Which of the following summarized subnets are not valid summaries that include subnets 10.1.55.0, 10.1.56.0, and 10.1.57.0, mask 255.255.255.0?" the two answers are 10.1.55.0 255.255.255.0 and 10.1.48.0 255.255.248.0. I can't figure out how to get this conclusion. what is the workout method.

negru_tudor

spruggie wrote: »

I've been trying to understand auto summarization by practicing questions, but i'm making no head way at all. a question I got is, "Which of the following summarized subnets are not valid summaries that include subnets 10.1.55.0, 10.1.56.0, and 10.1.57.0, mask 255.255.255.0?" the two answers are 10.1.55.0 255.255.255.0 and 10.1.48.0 255.255.248.0. I can't figure out how to get this conclusion. what is the workout method.

Well, 10.1.48.0 /21 goes from 10.1.48.0 /24 to 10.1.55.255 /24 and the other one doesn't work..where's the question from?

48 = 00110[000]

Toying around with the last 3 bits would take you from 48 (00110000) to 48 + 7 = 55 (00110111) on a 24 bit mask.

spruggie

the question required that I choose two answers.

A. 10.0.0.0 255.0.0.0
B. 10.1.0.0 255.255.0.0
C. 10.1.55.0 255.255.255.0
D. 10.1.48.0 255.255.248.0
E. 10.1.32.0 255.255.224.0

C & D were given as the two correct answers. I chose C & E,
C didn't look correct so I picked it but I couldn't figure out how to workout the other available choices
I just don't know what the working out method is

negru_tudor

spruggie wrote: »

the question required that I choose two answers.

A. 10.0.0.0 255.0.0.0
B. 10.1.0.0 255.255.0.0
C. 10.1.55.0 255.255.255.0
D. 10.1.48.0 255.255.248.0
E. 10.1.32.0 255.255.224.0

C & D were given as the two correct answers. I chose C & E,
C didn't look correct so I picked it but I couldn't figure out how to workout the other available choices
I just don't know what the working out method is

Well just look at the options that have a shorter mask than the ones you were asked about (they're all /24 bit). A and B fall from the start as the do include those 3 subnets (.55, .56 and .57). C is not a proper summary route for neither of these as it has a /24 mask so that's answer#1. D is indeed a summary but as posted above goes only as far as .55 so this is your answer#2.

If you look at E, you have:

32 = 001[00000] - you have 5 bits to toy around with, so on a /24 bit network you'd go from 10.1.32.0 /24 up until 10.1.63.255 /24 which includes those 3 so is not a proper answer for you.

spruggie

I'm starting to think that I understand the working out

224 equals a /19 value, giving me a 32 sub network range
32+32=64 so my range equals 32-63 as you showed

so 248 equals a /21 value, giving me an 8 sub network range
48+8=56 my range equals 48-55.

am I thinking along the right lines?

negru_tudor

spruggie wrote: »

I'm starting to think that I understand the working out

224 equals a /19 value, giving me a 32 sub network range
32+32=64 so my range equals 32-63 as you showed

so 248 equals a /21 value, giving me an 8 sub network range
48+8=56 my range equals 48-55.

am I thinking along the right lines?

yeap, that's it

spruggie

thanks a million, i've been practicing and its working