Subnetting
      It might be useful for the same administrative domain to contain 
       It might be useful for the same administrative domain to contain 
       several different networks:
       several different networks:
         Different link layer protocols without complex bridges
          Different link layer protocols without complex bridges
         Different administrative subdomains
          Different administrative subdomains
         Smaller tables on routers
          Smaller tables on routers
      But Class-B network addresses are running out, so we can’t waste 
       But Class-B network addresses are running out, so we can’t waste 
       them by assigning them where they won’t be densely populated
       them by assigning them where they won’t be densely populated
       And Class-C network addresses can only accommodate networks off 
        And Class-C network addresses can only accommodate networks off 
       255 machines, which is too small for even modest networks.
       255 machines, which is too small for even modest networks.
       The solution is to carve sub-networks (“subnets”) out of    
        The solution is to carve sub-networks (“subnets”) out of    
          larger Class-B networks
          larger Class-B networks
                                   
      Subnetting, cont. 
       If all routers needed to be aware of subnets, this would 
         If all routers needed to be aware of subnets, this would 
         drastically increase the size of routing tables. 
         drastically increase the size of routing tables. 
       To avoid this problem, routers outside of a network only 
         To avoid this problem, routers outside of a network only 
         know about whole networks
         know about whole networks
       Routers within a network however, need to know about the 
         Routers within a network however, need to know about the 
         subnets of their network.
         subnets of their network.
       This is achieved by having routers inside and outside of a 
         This is achieved by having routers inside and outside of a 
         network interpret the IP address differently. 
         network interpret the IP address differently. 
                                       
     Subnetting, cont.
     For compatibility, routers outside of a particular network must operate 
      For compatibility, routers outside of a particular network must operate 
      “exactly as before” – they make routing decisions based only on the 
      “exactly as before” – they make routing decisions based only on the 
      network number. 
      network number. 
     But, once within a network, the routers are all within the same 
      But, once within a network, the routers are all within the same 
      administrative domain, and can be configured to operate a little 
      administrative domain, and can be configured to operate a little 
      differently
      differently
     The portion of the address space that is normally used to identify the 
      The portion of the address space that is normally used to identify the 
      host number is partitioned into a subnet number and host number. This 
      host number is partitioned into a subnet number and host number. This 
      is done in much the same way as the IP address itself was partitioned 
      is done in much the same way as the IP address itself was partitioned 
      into a network number and host number.
      into a network number and host number.
     But, unlike the IP address as a whole, the number of bits used for each 
      But, unlike the IP address as a whole, the number of bits used for each 
      is not fixed. Instead it is determined by the subnet mask.
      is not fixed. Instead it is determined by the subnet mask.
                               
     Subnetting, cont.
      Each host can be part of one subnet. This subnet has a subnet number and a 
       Each host can be part of one subnet. This subnet has a subnet number and a 
       subnet mask.
       subnet mask.
      For example 128.96.64.0 is a subnet of network 128.96
       For example 128.96.64.0 is a subnet of network 128.96
      The network number is parsed exactly as it is before, based on the packet’s 
       The network number is parsed exactly as it is before, based on the packet’s 
       prefix and the class of the address.
       prefix and the class of the address.
      Within a particular address, the subnet mask is used to separate the subnet 
       Within a particular address, the subnet mask is used to separate the subnet 
       number from the host number. 
       number from the host number. 
      The subnet mask is the same length as an IP address and contains a 1 in every 
       The subnet mask is the same length as an IP address and contains a 1 in every 
       position that is part of the subnet number (including network number) and a 0 
       position that is part of the subnet number (including network number) and a 0 
       everywhere else. It might also need to contain additional 1s to distinguish it 
       everywhere else. It might also need to contain additional 1s to distinguish it 
       from other subnets. 
       from other subnets. 
      By ANDing the subnet mask with the IP address, the host number is 
       By ANDing the subnet mask with the IP address, the host number is 
       eliminated, leaving only the subnet number. In this way a host knows if a 
       eliminated, leaving only the subnet number. In this way a host knows if a 
       destination is on its own network.
       destination is on its own network.
                                
      Subnetting, cont. 
   Subnets and subnet masks
  Subnet Num:  128. 96.  0.  0   10000000.01100000.00000000.00000000
  Subnet Mask: 255.255.255.  128 11111111.11111111.11111111.10000000
  ~127 hosts
  Notice that the network mask hides host numbers:
   IP Address:  128. 96.  0.  82  10000000.01100000.00000000.01010010
   Subnet Mask: 255.255.255.  128 11111111.11111111.11111111.10000000  
   Subnet Num:  128. 96.  0.  128 10000000.01100000.00000000.0d0000000