CSIS 3723

 We need to create some logic to the environment  We want to keep like devices together  We want to make money leasing the use of the space  Security

 When designing the Internet it was decided that not all devices would need or want to be addressable from the Internet but the will still need to communicate using the network  Because of this private address space was created  These addresses are not accessible from the Internet without the network administrator doing something to give them a Internet address (NAT)  These addresses can be accessible in your intranet (corporate space)

 RFC 1918 defines these ◦ ( /8) ◦ – ( /12) ◦ – ( /16)  These are the IP address spaces that can be used internally in an enterprise

 RFC states a “link local” block ◦ – ( /16) ◦ To be used when a device can not get an IP address through DHCP  Also reserves lowest Class B ◦ ( /16) ◦ Not able to be used under old class system but can be assigned to someone  Also defines loop back space (RFC 1700) ◦ – ( /8) ◦ Used for a machine to communicate internally  Also defines multicast address space (RFC 5771) ◦ – ( /4)  So you should never use these IP address spaces!

/ Internet /24

 What are the IP addresses for the subnet /24?  through  Anything after the 24 th most significant bit can change and be in the same subnet

 We use the CIDR as a binary number  Every most significant bit is a one the rest are zero  So a /24 would be: Subnet Mask

 What would subnet mask be for /20? = 255

/ IP Address Subnet Mask Logical AND Logical AND Bitwise AND is used. Logical AND is done on each bit between the IP address and the subnet mask. If the result matches the network it is in the same subnet Network Destination Address Network

/ IP Address Subnet Mask Logical AND Network Destination Address Network No match different subnet

 If we look at just one octet we see a pattern MaskBinaryRatio : : : : : : : : :1

/24 If I start with: This is the last octet: : :256 If we change it to: We would get two networks: / /25

Lets look at what happens when the number change in the last octet MaskLast Octet of IP address As long as this bit does not become a one in the IP address it is in the first subnet First Subnet Second Subnet

What if an ISP owns a block of IP addresses like: ISP's block /20 If I had 8 customers that want to buy subnets how could I change the subnet mask to get 8 subnets? / Each place I move I get a multiple of two

To get 8 in binary I would need 3 binary numbers = 7 ??? We start count from zero

ISP's block /20 Organization /23 Organization /23 Organization /23... ….. …. …. Organization /23

Organization /24 Organization /24 Organization /24... ….. …. …. Organization /24 What if I needed 11 subnets??? ISP's block /20 We need to create 16 subnets to get 11

What if I needed subnets that can have 56 hosts??? ISP's block /20 Where can we move from the right to get a number larger the 56 ??? = 63 (plus one for zero) is 64

Organization /26 Organization /26 Organization /26... ….. …. …. Organization /26 What if I needed subnets that can have 56 hosts??? ISP's block /20 We need to create 64 subnets each having 64 IP addresses

 On the subnet /26  We only get 61 usable IP address  One is used for the network ◦ not used  One must be used for the router interface on the subnet ◦ is usual used as the router IP address but does not have to (could be or any other host IP address)  One must be used for the broadcast address ◦ Is always the last IP address in the subnet ( )