Address Classes

Publicly assignable #IPv4 Address are divided into 5 classes to accommodate the increasing demands. See the table below:

Classes	First Octet Range	Network Bits	Host Bits
A	1-126	8	24
B	128-191	16	16
C	192-223	24	8
D	224-239
E	240-255

Note: Class D is utilised for multicast, so there is no need to distinguish between network bits and host bits.

Note: Class E is reserved for research and development (R&D).

Note: Octet 127 can’t be seen in the table with a reason: it is reserved for local addressing. In fact, your local machine will have an address of 127.0.0.1 by default.

The network usually reserve the first address and last address of the octet for network identification (which network is it) and broadcast (local) respectively. From here, we can know the total number of assignable hosts in the network by the formula \(2^{BL} - 2\) where \(BL\) denoted the number of assigned host bits (\(2^{BL}\) is the total number of hosts including the network address and broadcast address). The total number of hosts calculation can be more sophisticated with the introduction of #Subnetting. We also get a magic number by dividing \(2^{BL}\) by 256 until there is a remainder, which will be the gap between assignable subnets.

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Subnetting

Subnetting is a networking technique used to separate hosts within a network which involves sliding a portion of the #host bits. The network that doesn’t implement this technique is called a flat network.
Static Routing

Static Routing is a static way to configure the #Routing Table of a Router#. It is static as not reactive to external changes such as topology changes. Usually, this is done manually by the network administrator in order to have some sort of control over route configuration. One particular example is using static route in a stub network, where there is only one route for the network to be accessed. We could simplify the network routing by using route summarisation which represents all similar routes with a single network (using network class’s default route# but smaller subnet mask is possible) if the routes share a single exit interface.
Routing Information Protocol (RIP)

RIP only propagate the network address of known routes. There is no need to include subnet mask as part of the routing information since it could determine the subnet mask by just looking at the Address Classes default subnet masks. The boundary Router (the entry router to the RIP network) will summarise the RIP subnets from the internal network to the external network. This reduces the size of the Routing Table and resulted in a faster lookup in the routing table (single route can represent multiple routes). However, it comes at a cost of not able to support discontiguous network. Furthermore, if the subnet mask doesn’t match the class or a subnet of the class, RIP will not advertise the route to the other RIP enabled routers.
IPv6 Address

The notation for IPv6 Address is quite simple. There is no Address Classes for it. It is divided into 8 fields of 16 bits in Hex. The first 64-bit is for the network ID, and the rest 64-bit is for the host ID. Network ID is further segmented into prefix chunks. A typical example of it is like this:
IPv4 Address

IPv4 Address can be divided into two parts: network and host bits. Depending on the Address Classes#, the size of the network and host bits can be varying, plus with the introduction of the Subnetting#, host bits can be quite small.
Fix-Length Subnet Masking

Fix-Length Subnet Masking is a #Subnetting technique utilises the default subnet mask of an Address Class#. All non-reserved address classes has a default subnet mask, shown in the following table:
Classless Routing Protocols

Classless Routing Protocols are the Routing Protocols that don’t follow the Address Classes subnet masking instead favour the Variable-Length Subnet Masking (VLSM)#. All protocols will include network mask when sending the routing information to other Network Device. It is aimed to replace Classful Routing Protocols.
Classful Routing Protocols

Classful Routing Protocols are the Routing Protocols# that follow the Fix-Length Subnet Masking#. However, as the networking technologies advanced, most Classful Routing Protocols are phased out in favour of Classless Routing Protocols due to the limitation of Classful Routing Protocols to travel to different Address Classes since the routing update doesn’t include the network mask.

#networking