Continue with working with IP Addresses

Ø Subnet mask

v An IP address has 2 parts:

      ·        The Network identification.

      ·        The Host identification.

v Frequently, the Network & Host portions of the address need to be separately extracted.

v In most cases, if you know the address class, it’s easy to separate the 2 portions.

v With the rapid growth of the internet & the ever-increasing demand for new addresses, the standard address class structure has been expanded by borrowing bits from the Host portion to allow for more Networks.

v Under this addressing scheme, called Subnetting, separating the Network & Host requires a special process called Subnet Masking.

v The subnet masking process was developed to identify & extract the Network part of the address.

v The function of a subnet mask is to determine whether an IP address exists on the local network or whether it must be routed outside the local network.

v However, if network address don’t matches the local network ID, the message must be routed outside the local network.

v The process used to apply the subnet mask involves Boolean algebra to filter out non-matching bits to identify the network address.

v Boolean algebra is a process that applies binary logic to yield binary results.

v Working with subnet masks, you need only 4 basic principles of Boolean Algebra:

·        1 and 1 = 1

·        1 and 0 = 0

·        0 and 1 = 0

·        0 and 0 = 0

v The process of combining binary values with Boolean Algebra is called Anding.

Ø Default Standard Subnet mask

v Subnet masks apply only to Class A, B or C IP addresses.

v The subnet mask is like a filter that is applied to a message’s destination IP address.

v If a destination IP address is 206.175.162.21, we know that it is a Class C.

v We also know that the default standard Class C subnet mask is: 255.255.255.0

v When these two binary numbers (the IP address & the subnet mask) are combine using Boolean Algebra, the Network ID of the destination network is the result.

v The result is the IP address of the network. If this address is the same as the local network, the message is for a node on the local network.

Ø Routing IP Addresses

v When you build a network, you need to figure out how many network IDs your network requires.

v There’s no hard & fast rule on how you should dole out your allotted IP addresses.

v Commonly, though, the lowest numbers (1 through 10) are assigned to routers & servers but how you assign addresses is strictly up to you & your network policies & guidelines.

Ø What is Subnetting ?

v Subnetting is the foundation underlying the expansion of both Local Networks & the Internet in today’s world.

v Subnetting has become essential knowledge for the Administrator of any network.

v There are 2 fundamental reasons why subnetting has so much importance in today’s networking environment:

 ·   The world is running out of available IP addresses. Subnetting helps allocate the IP addresses efficiently.

 ·  Subnetting reduces the size of the routing tables stored in routers. Subnetting hides the internal network organization to external routers and thus simple routing.

v There are only 3 usable IP address classes:

·        Class A

·        Class B

·        Class C

v Class A networks have the highest number of available hosts.

v Class C networks have the fewest number of hosts.

Ø Subnetting Networks ID

A 3-step example of how the default Class A subnet mask is applied to a Class A address:

v Default Class A subnet mask (255.0.0.0) is AND operation with the Class A address (123.123.123.1) using Boolean Algebra, which results in the Network ID (123.0.0.0) being revealed.

v The default Class B subnet mask (255.255.0.0) strips out the 16-bit network ID & the default Class C subnet mask (255.255.255.0) strips out the 24-bit network ID. 

Ø Subnetting, Subnet, Subnet Mask

v Subnetting, a subnet & a subnet mask are all different.

v Subnetting is the process of dividing a network & its IP addresses into segments, each of which is called a subnetwork or subnet.

v The subnet mask is the 32-bit number that the router uses to cover up the network address to show which bits are being used to identify the subnet.

Ø Subnetting

v A network has its own unique address, such as a Class B network with the address 152.20.0.0 which has all zeroes in the host portion of the address.

v From the basic definitions of a Class B network & the default Class B subnet mask, you know that this network can be created as a single network that contains 65,534 individual hosts.

v Subnetting does not improve the available shared bandwidth. But, it cuts down on the amount of broadcast traffic generated over the entire network as well.

Ø Benefits of Subnetting

v Reduced network traffic:

One network will not access the data of other network without the use of router. Thus we can reduce the amount of data remain in one network.

v It provides security:

In larger companies, employees must be able to communicate with other employees from that department. Subnetting allows for the department to have its own subnetwork.

v It allows organizing resources:

A company can have several departments or types of resources: sales, customer care, IT, executive, research.With subnetting, these resources can be organized within the larger network.

v Example of subnetting: when the network administrator divides the 152.20.0.0 network into 5 smaller networks –152.20.1.0, 152.20.2.0, 152.20.3.0, 152.20.4.0 & 152.20.5.0 – the outside world 152.20.4.0 & 152.20.5.0 – the outside world stills sees the network as 152.20.0.0, but the internal routers now break the network addressing into the 5 smaller subnetworks.

v In the example, only a single network address is used to reference the network & instead of 5 network addresses, only one network reference is included in the routing tables of routers on other networks.

v The key concept in subnetting is borrowing bits from the host portion of the network to create a subnetwork.

v Rules govern this borrowing, ensuring that some bits are left for a Host ID.

v The rules require that two bits remain available to use for the Host ID & that all of the subnet bits cannot be all 1s or 0s at the same time.

v For each IP address class, only a certain number of bits can be borrowed from the host portion for use in the subnet mask.

v To subnet a Class A network, you need to borrow a sufficient number of bits from the 24-bit host portion of the mask to allow for the number of subnets you plan to create, now & in the future.

v A sample of subnet mask options available for Class A addresses.

v All subnet masks contain 32 bits; no more, no less.However a subnet mask cannot filter more than 30 bits. This means 2 things:

·        One, that there cannot be more than 30 ones bits in the subnet mask.

·        Two, that there must always be at least 2 bits available for the Host ID.

v Remember that the addresses with all ones (broadcast address) & all zeroes (local network) cannot be used as they have special meanings.

v The table “Class A Subnet Masks” is similar to the tables used for Class B & Class C IP addresses & subnet masks. The only differences are that you have fewer options (due to a fewer number of bits available).

v A sample of the subnet masks available for Class B networks.

v A list of the subnet masks available for Class C networks.

v To determine the number of subnets & hosts per subnet available for any of the available subnet masks, 2 simple formulas to calculate these numbers:

       

Ø Short Broadcast

v A broadcast is a message that every node on a network or subnetwork receives & examines.

v Cisco IOS supports 2 different types of broadcast messages:

·        Flooded

·        Directed

v Generally speaking, routers do not propagate broadcasts, which is one of the benefits of installing a router in the first place.

v Flooded broadcasts (those with the nominal broadcast address of 255.255.255.255) are not forwarded by the router & are considered local traffic only.

v Directed broadcasts, which contain all 1’s in the Host portion of the IP address, are addressed to a specific sub network & are allowed to pass.