Classless addressing differs from Classful addressing because it allows the subnet mask to use bits from the preceding octet(s) to increase or decrease the bits used for the Network ID and Host ID respectively.

CIDR (Classless Interdomain Routing)

Since you really need the subnet mask to determine the number of bits in the Network and Host IDs a shorthand method was created to allow you to determine the number of bits used for the network ID using slash (/) notation. CIDR notation takes the host IP address and follows it with a slash and number like /24 following the IP address. Here’s an example the address and subnet could be written as 192.168.0.1-255.255.255.0 indicating a class C network using 24 bits for the network portion. This can be abbreviated to 192.168.0.1/24 with CIDR notation. Should an additional network bit be required you can borrow that bit with 192.168.0.1/25 creating two additional subnets. These would have the network ID 11000000.10101000.00000000.00000000 or 11000000.10101000.00000000.10000000 with 16 bits available for the host ID. Here’s a brief graphic that shows the relationship of CIDR and the binary representation of a /32 network.

VLSM

So far creating subnets was a matter of dividing the new subnets into smaller equal portions. In practice, this could prove to be an inefficient approach as some subnets may waste addresses while others may need more. Variable Length Subnet Masks (VLSM) was developed to allow subnets to be further divided to more efficiently match the needs of the network. You create the largest subnet first then “subnet the subnet” from there.

CIDR notation (IPv4 vs. IPv6)

IPv6 Networks differ substantially from IPv4 as they are classless, do not use subnet masks and a single subnet can support 18,446,744,073,709,551,616 on a Residential – /64 network.

Address assignments

DHCP

In order to manage IP addressing and configuration on network clients, a DHCP (Dynamic Host Configuration Protocol) server is used. The DHCP server automatically provides all of the configuration information to clients while preventing address duplication. The process will be familiar to those using a residential gateway or SOHO router. The DHCP server is provided a range of addresses (scope) available for it to provide to clients. Addresses are not permanently or statically assigned to clients by the server. They are leased. When the lease expires it can be renewed by the client or assigned to a different client. The configuration information provided to the client includes the lease time, when the lease was obtained and when it expires, the default gateway address and primary and secondary DNS server addresses. This process is transparent to the user. The DHCP server listens on port 67 for the client to request DHCP service. When the request is received the server responds on port 68 with the configuration.

Since the DHCP assigned IP address is not static you may obtain a different address each time you log on to the network. This works well for standard clients but some devices, printers, for example, require a permanent address to be consistently accessible to the clients. In this case, the server is configured to use the same address for that device using an IP reservation. When the server sees the MAC address of the printer it will provide the same address to it consistently. Static IP addresses can be used along with DHCP. To avoid conflicts the statically assigned addresses should be excluded from the servers address range using an IP exclusion.

DHCPv6

DHCPv6 handles dynamic address assignment much the same way DHCPv4 does with the exception of the ports used. DHCPv6 listens on port 546 and responds on port 547. The IPv6 process is known as autoconfiguration and assigns addresses using FE80::/64 as the first 64 bits. The host portion of the address can be assigned randomly or it can use the EUI-64 method shown below. 

Static

When you manually set the IP address it is known as a static IP address. Statically assigned addresses are used when servers, printers, and other network devices need to be consistently available to the network nodes at the same address. Static addressing increases administrative overhead but is necessary for devices that provide network services. Otherwise, DHCP is the preferred method.

APIPA

Automatic Private IP Addressing (APIPA) is an addressing method used when a DHCP client is unable to obtain a lease from the server. APIPA takes over and assigns addresses in the 169.254.0.1 to 169.254.255.255 IP range. This configuration will remain in place until a DHCP server is located.

EUI64

In IPv6 addressing the first 64 bits represent the network portion of the address. The remaining 64 bits can be randomly generated or they can be based on the device’s MAC address. Since a MAC address is 48 bits more host bits are needed. The 48 bit MAC address uses EUI-64 (Extended Unique Identifier-64) to be converted to meet the 64-bit standard. This calls for an additional 16 bits to be added and the inversion of the seventh bit.

That’s it for objective 1.4. Good luck on the test!