Category Archives: IP Addressing

Practical OSI Layers – Part 2

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In the previous post, we took a look at the basics of the OSI and TCP/IP model layers. In this post we are going to take a closer look at end to end routing of a packet, and the interaction between layers 2 and 3 as a packet is passed between routers to it’s ultimate destination. The following is the general process to route between endpoints. Some operating systems may behave a little different that described, but this is the general process.

This post will use this network, with a telnet session from R1 to R3. R1 is connected to R2 via a serial link running PPP, and R2 is connected to R3 via an Ethernet segment. Continue reading →

Basic Subnetting Trick

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This is good for Cisco exams, and work, if you ever need to figure out subnets on paper. 

To figure out the valid addresses in a subnet: 
Take the octet that is not 0 or 255, for example starting with 255.255.224.0, take 224, and subtract that value from 256 (256 – 224 = 32). Now make a chart starting at 0, and adding the value from that last step with each line, up to 256 (the note board I got for the CCNA had gridlines, which made it even easier) 

0 
32 
64 
96 
128 
160 
192 
224 
256 


Now, leaving room in between, write on each line the value of the line below, minus 1 

 

 
  0       31 
 32       63 
 64       95 
 96       127 
128      159 
160      191 
192      223 
224      255 
256 


The values on the left are the valid subnet addresses, and the right is the broadcast address. 

If you want, now fill in the valid ranges in between 

 

Code:
  0         1-30          31 
  32      33-62          63 
  64      63-94          95 
  96      97-126        127 
128     127-158        159 
160     161-190        191 
192     192-222        223 
224     225-254        255 
256 


There you go, all your addresses, with subnet and broadcast addresses.