Ip Addressess

• Each address is 4 bytes long.
• Usually grouped in 4 sets of a byte each.
• 147.64.242.52
• Are issued to individuals and institutions by an ISP.
• Originally the network was divided into three "classes"
  • Class A, or addresses with the first octet 1-126
  • 10.x.x.x for example
  • Class B or addresses 128-191 as the first octet
  • The second octet is used as well.
  • 147.64.x.x for example
  • Class C or addresses with 192-223 in the first octet
  • 198.23.42.x
  • The second and third octets are part of the address as well
  • Other (class D and E) had special uses but were very small.
• Some have special meaning
  • 127.0.0.1 is the loopback address, hostname loopback
  • 10.0.0.0 is the class A private network
  • 172.16.0.0 is the class B private network
  • 192.168.0.0 is the class C private network
  • The last three should not be routed outside your network.
  • And they will never be assigned
• Each machine on the net must have at least one unique ip address.
• Subnet Masks
  • Are used to divide an ip address into two parts
    • The network part
    • The address of a machine.
    • By the way, the machine can not be all 0s or all 1s
  • A 1 in a netmask means that bit is assigned to the network address
  • So a mask of 255.255.255.0
    • Means that there are 24 network bits and 8 machine bits.
    • 131.123.32.102
    • 102 is the machine,
    • 131.123.32 is the network
    • In the classical model, a class C would have 254 machines (no 0 or 255)
    • A class A netmask is 255.0.0.0
    • A class B netmask is 255.255.0.0
    • A class C netmask is 255.255.255.0
• But if you have a class B, there is no reason you can't further subdivide your network.
  • This allows you to reduce traffic on a network
  • And distribute control of your network.
  • You just need a way to route between these networks.
• CIDR (classless inter-domain routing) notation
  • It is hard to keep track of subnet masks that don't end on an octet boundary, so the custom is to add a /XX to represent the number of bits in the netmas.
  • We use 255.255.254.00/23
  • Which is 11111111.11111111.11111110.00000000
  • This means we have 2⁹-2 or 510 addresses.
  • If we have a subnet mask of 255.255.255.192/26
  • FF.FF.FF.1100 0000
  • And our address is 199.43.58.x
  • We have further divided our network into four subnets
  • 199.43.58.0/26, (.00) with machines 199.43.58.1 to 199.43.58.62
  • 199.53.58.64/26,(.01) with machines 199.43.58.65 to 199.43.58.126 (+64)
  • 199.53.58.128/26, (.10) with machines 199.43.58.129 to 199.43.58.192 (+128)
  • 199.53.58.192/26, (.11) with machines 199.43.58.193 to 199.43.58.255 (+192)
  • Or we have four networks, with 62 machines each.
• Use of netmasks and addresses
  • When a machine sees a packet
  • It looks at the IP address
  • If the network portion of the address (as determined by the machine's netmask) matches the network portion in the packet
  • It further looks to see if the machine part matches its machine part, or if it is is all 1s.
  • In either case, it receives the packet.
  • If a router encounters a packet with an ip address for the network it is on, it does nothing.
  • If a router encounters a packet with an ip address for another network, it does its best to send the packet on.