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IP Addressing / CIDR
Classless Inter-Domain Routing
• Each node on the same network must have a unique IP address.
• Today, the most commonly used version of IP is IP version 4 (IPv4),
• Its addresses are in such high demand that supply has started to run
out.
• IP version 6 (IPv6) was created to address this shortage.
– (IPv6 also has many security features built into it that are not part of IPv4.)
• IPv4 uses 32 bits for its addresses,
• IPv6 uses 128 bits;
– IPv6 provides more possible addresses with which to work.
• Each address has a host portion and a network portion,
• Addresses are grouped into classes and then into subnets.
• The subnet mask of the address differentiates the groups of
addresses that define the subnets of a network.
IPv4 address classes
Class
Range of first octet
Number of networks
Number of nodes per network
A
0 - 127
128
16,777,216
B
128–191
16,384
65,536
C
192–223
20,97,152
256
D
224-239
Multicast
E
240-255
Reserved for Research / future use
• All nodes connected to the network can have different host addresses but a common
network address.
• The host address identifies every individual node, whereas the network address is the
identity of the network they are all connected to
• Each IPv4 Address is 32 Bits long
• Depending on the class, X bits will identify the Network and Y
identifies the Host
• Example – Class A
1 1 1 1 1 0 1 0 0 0 0 1 0 1 0 0 0 0 0 1 1 0 0 0 0 0 0 1 0 1 0
• 125.14.12.10
• In this example 8 Bits identify the Network and 16 bits Identify the
host
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Subnet
• A subnet is created from the host portion of an IP address to
designate a "sub" network.
• This allows us to further break the host portion of the address into
two or more logical groupings.
• A network can be logically partitioned to reduce administration
headaches, traffic performance, and potentially security.
• Network subnetting is not physical, it is logical.
• A subnet mask defines smaller networks inside a larger network
• Subnetting allows large IP ranges to be divided into smaller, logical,
and more tangible network segments.
• If the traditional subnet masks are used,
• they are referred to as classful or classical IP addresses.
• If an organization needs to create subnets that do not follow these
traditional sizes, then it would use classless IP addresses.
• This just means a different subnet mask would be used to define the
network and host portions of the addresses.
Starting and ending numbers
• Subnets have a beginning and an ending,
• The beginning number is always even and the ending number is always odd.
• The beginning number is the "Network ID" and the ending number is the "Broadcast
ID."
• You can’t use these numbers because they both have special meaning with
special purposes.
• The Network ID is the official designation for a particular subnet,
• The ending number is the broadcast address that every device on a subnet
listens to.
• Anytime you want to refer to a subnet, you point to its Network ID and its
subnet mask, which defines its size.
• Anytime you want to send data to everyone on the subnet (such as a
multicast), you send it to the Broadcast ID.
Classless inter-domain routing (CIDR)
• After it became clear that available IP addresses were running out as
more individuals and corporations participated on the Internet,
classless interdomain routing (CIDR) was created.
• A Class B address range is usually too large for most companies, and a
class C address range may be too small,
• CIDR provides the flexibility to increase or decrease the class sizes as
necessary.
• CIDR is the method to specify more flexible IP address classes.
CIDR notation
• CIDR notation is a syntax for specifying IP addresses and their
associated routing prefix.
• CIDR appends a slash character to the address and the decimal
number of leading bits of the routing prefix,
• The /24 or whatever decimal number follows the slash ‘/’ identifies
the networking portion of the address leaving the remaining as the
host identifier
• e.g., 192.168.2.0/24 for IPv4,
CIDR Example
• 192.168.100.0/24 represents an IPv4 address and its associated
routing prefix 192.168.100.0, or equivalently, its subnet mask
255.255.255.0
• /24 relates to the amount of 1’s in the subnet mask
• 11111111 11111111 11111111 00000000
• =255.255.255.0
• 24 bits identify the host portion, 8 bits identify the node
CIDR Example
• 192.168.100.0/16
• 11111111 11111111 00000000 00000000
• 255.255.0.0
• 16 bits identify the network portion, 16 bits identify the node
CIDR Example
• 192.168.100.0/22
• 11111111 11111111 11111100 00000000
• 255.255.252.0
• 22 Bits identify the Network portion, 10 bits identify the node
The mask length
refers to the /number
in the CIDR notation
Note the repeating pattern
it will come in handy to
remember eight special
numbers that reoccur when
dealing with subnet masks.
They are
255, 254, 252, 248, 240,
224, 192, and 128.