IP
|
Description:
Internet Protocol
The network layer
protocol in the TCP/IP stack that offers a connectionless
internetwork service. IP provides features for addressing,
type-of-service specification, fragmentation and reassembly, and
security. Documented in RFC 791. |
subnet address
|
Description:
A portion of an IP
address that is specified as the subnetwork by the subnet mask.
See also IP address, subnet
mask, and subnetwork. |
IP address
|
Description:
1) A 32-bit address
assigned to hosts that use TCP/IP. An IP address belongs to one of
five classes, A, B, C, D, or E. An IP address is written as 4 octets
separated with periods, referred to as dotted decimal format. Each
address consists of a network number, an optional subnetwork number,
and a host number. The network and subnetwork numbers together are
used for routing, while the host number is used to address an
individual host within the network or subnetwork. A subnet mask is
used to extract network and subnetwork information from the IP
address. Also called an Internet address.
2) The command used to
establish the logical network address of an interface.
See also IP and subnet mask.
|
![]()
src="pagina_nueva_11_archivos/image001.gif" v:shapes="_x0000_s1025">
To create the subnetwork structure,
host bits must be reassigned as network bits. This is often referred to as
‘borrowing’
bits. However, a more accurate term would be ‘lending’
bits. The starting point for this process is always the leftmost host bit, the
one closest to the last network octet.
Subnet addresses include the Class A,
Class B, and Class C network portion, plus a subnet field and a host field. The
subnet field and the host field are created from the original host portion of
the major IP address. This is done by re-assigning bits from the host portion to
the original network portion of the address.
-
The ability to divide the original host portion of the address into the new
subnet and host fields provides addressing flexibility for the network
administrator.
In addition to the need for
manageability, subnetting enables the network administrator to provide broadcast
containment and low-level security on the LAN. Subnetting provides some security
since access to other subnets is only available through the services of a
router. Further, access security may be provided through the use of access
lists. These lists can permit or deny access to a subnet, based on a variety of
criteria, thereby providing more security. Access lists will be studied later in
the curriculum. Some owners of Class A and B networks have also discovered that
subnetting creates a revenue source for the organization through the leasing or
sale of previously unused IP addresses.
Subnetting is an internal function of
a network. From the outside, a LAN is seen as a single network with no details
of the internal network structure. This view of the network keeps the routing
tables small and efficient. Given a local node address of 147.10.43.14 on subnet
147.10.43.0, the world outside the LAN sees only the advertised major network
number of 147.10.0.0. The reason for this is that the local subnet address of
147.10.43.0 is only valid within the LAN where subnetting is applied.
Selecting the number of bits to use
in the subnet process will depend on the maximum number of hosts required per
subnet. An understanding of basic binary math and the position value of the bits
in each octet is necessary when calculating the number of subnetworks and hosts
created when bits were borrowed.
The last two bits in the last octet,
regardless of the IP address class, may never be assigned to the subnetwork.
These bits are referred to as the last two significant bits. Use of all the
available bits to create subnets, except these last two, will result in subnets
with only two usable hosts. This is a practical address conservation method for
addressing serial router links. However, for a working LAN this would result in
prohibitive equipment costs.
The subnet mask gives the router the
information required to determine in which network and subnet a particular host
resides.
The subnet mask is created by using binary ones in the network bit positions.
The subnet bits are determined by adding the position value of the bits that
were borrowed. If three bits were borrowed, the mask for a Class C address would
be 255.255.255.224.
This mask may also be represented, in the slash format, as /27. The number
following the slash is the total number of bits that were used for the network
and subnetwork portion.
To determine the number of bits to be
used, the network designer needs to calculate how many hosts the largest
subnetwork requires and the number of subnetworks needed. As an example, the
network requires six subnetworks of 25 hosts each. A shortcut to determine how
many bits to reassign is by using the subnetting chart.
By consulting the row titled ”Usable
Subnets”, the chart indicates that for six usable
subnets three additional bits are required in the subnet mask. The chart also
shows that this creates 30 usable hosts per subnet, which will satisfy the
requirements of this scheme. The difference between usable hosts and total hosts
is a result of using the first available address as the ID and the last
available address as the broadcast for each subnetwork. Borrowing the
appropriate number of bits to accommodate required subnetworks and hosts per
subnetwork can be a balancing act and may result in unused host addresses in
multiple subnetworks. The ability to use these addresses is not provided with
classful routing. However, classless routing, which will be covered later in the
course can recover many of these lost addresses.
The method that was used to create
the subnet chart can be used to solve all subnetting problems.
This method uses the following formula:
Number of usable subnets = two to the
power of the assigned subnet bits or borrowed bits, minus two. The minus two is
for the reserved addresses of network ID and network broadcast.
2 power of borrowed bits
- 2 = usable subnets
2 3 - 2 = 6
Number of usable hosts = two to the
power of the bits remaining, minus two (reserved addresses for subnet id and
subnet broadcast).
2 power of remaining host
bits - 2 = usable hosts
2 5 - 2 = 30
Once the subnet mask has been
established it then can be used to create the subnet scheme. The chart in Figure
is an example of the subnets and addresses created by assigning three bits to
the subnet field. This will create eight subnets with 32 hosts per subnet. Start
with zero (0) when numbering subnets. The first subnet is always referenced as
the zero subnet.
When filling in the subnet chart
three of the fields are automatic, others require some calculation. The
subnetwork ID of subnet zero is the same as the major network number, in this
case 192.168.10.0. The broadcast ID for the whole network is the largest number
possible, in this case 192.168.10.255. The third number that is given is the
subnetwork ID for subnet number seven. This number is the three network octets
with the subnet mask number inserted in the fourth octet position. Three bits
were assigned to the subnet field with a cumulative value of 224.
The ID for subnet seven is 192.168.10.224. By inserting these numbers,
checkpoints have been established that will verify the accuracy when the chart
is completed.
When consulting the subnetting chart
or using the formula, the three bits assigned to the subnet field will result in
32 total hosts assigned to each subnet.
This information provides the step count for each subnetwork ID. Adding 32 to
each preceding number, starting with subnet zero, the ID for each subnet is
established.
Notice that the subnet ID has all binary 0s in the host portion.
The broadcast field is the last
number in each subnetwork, and has all binary ones in the host portion. This
address has the ability to broadcast only to the members of a single subnet.
Since the subnetwork ID for subnet zero is 192.168.10.0 and there are 32 total
hosts the broadcast ID would be 192.168.10.31. Starting at zero the 32nd
sequential number is 31. It is important to remember that zero (0) is a real
number in the world of networking.
The balance of the broadcast ID
column can be filled in using the same process that was used in the subnetwork
ID column. Simply add 32 to the preceding broadcast ID of the subnet. Another
option is to start at the bottom of this column and work up to the top by
subtracting one from the preceding subnetwork ID.
![]()
src="pagina_nueva_11_archivos/image003.jpg" v:shapes="_x0000_s1026">
This is the way how subnetting works
![]()
src="pagina_nueva_11_archivos/image005.jpg" v:shapes="_x0000_s1027">
Domain
|
Description:
Networking system
developed by Apollo Computer (now part of Hewlett-Packard) for use
in its engineering workstations. |
|
ICMP
|
Description:
Internet
Control Message Protocol
The network
layer Internet protocol that reports errors and provides
other information relevant to IP packet processing.
Documented in RFC 792.
|
|
enlace
