User's Manual
Issue 2.0
TimeLord Master Clocks
Operating and Installation Instructions
9-2
There are five different classes of address:
Class A - 1.x.x.x - 126.x.x.x
These addresses have a 8 bit network number and a 24 bit
host number addresses. Each class A network can have
16,777,214 hosts.
Class B - 128.1.x.x - 191.254.x.x
These addresses have a 16 bit network number and a 16 bit
host addresses. This means that each class B network can have
65,354 hosts.
Class C - 192.0.1.x - 223.255.254.x
These addresses have a 24 bit network number and an 8 bit
host addresses. This means that each class C network can
have 254 hosts.
Class D - 224.0.0.0 - 239.255.255.255
Class D network addresses are used for multicasting, which is
where information sent from one address can be sent to many
different hosts simultaneously.
Class E - 240.0.0.0 - 254.255.255.255
Class E network addresses are used for experimental
purposes.
For each unique network number, the base address of the
range (i.e. host number zero) is known as the network address
and is not assigned to a host. The top address of the range is
(i.e. host number set to all ones) the broadcast address which
is used as the address for sending to all hosts on the same
network number simultaneously.
Subnet Addressing
Subnet addressing allows us to split one IP network address
into several smaller physical networks known as subnets. This
is especially useful with Class A and Class B addresses where
there are very large numbers of network hosts with the same
network address.
A subnet is created by splitting the host address part of the IP
address into two, the first being subnet address and the
second host ID.
In the class B example shown above the address has been
subnetted into 254 subnets, each with a possible 254 hosts.
Subnet Mask
The subnet mask is a 32 bit number which specifies how many
bits of the IP address are for the network / subnet address and
how many are for the host address.
For the class B subnet example used above the subnet mask
would be as follows:
The following table details a few typical subnet masks and the
network classes / subnets they describe.
Private IP addresses
With the massive growth of the Internet there was a real
danger that the IANA would run out of IP addresses. To avoid
this situation three blocks of IP addresses have been reserved
for use in private networks. It is recommended that on an
organisation’s TCP/IP networks use these private network
numbers for their networks and make use of a technology like
NAT (Network Address Translation) to allow their users access
to the Internet.
10.0.0.0 - 10.255.255.255
172.16.0.0 - 172.31.255.255
192.168.0.0 - 192.168.255.255
ARP & MAC
When one device (A) wants to send data to another device (B)
on the same Ethernet LAN it must convert the IP address of the
destination device to the Ethernet Media Access Control (MAC)
address of that device. Each Ethernet device has a globally
unique 48 bit MAC address which is assigned at time of
manufacture.
If device A does not have B’s MAC address an Address
Resolution Protocol (ARP) request is broadcast onto the local
Network. The ARP request contains the IP address of device B,
so device B is the only device to respond. Device B sends its
MAC address to device A and it is stored in device A’s ARP
table.
Device A can now send data directly to device B.
Gateway Address
The gateway address is used when a device on a network
sends data to another device that is connected to a different
network segment. The gateway device is typically a router
connecting two different networks together.
When one device sends data to another device the TCP/IP
software checks to see if the destination network address is
the same as its own network address. If the network address
is the same the data is transmitted normally, using ARP to
resolve the destination MAC address if required.
If the network address is different the data is forwarded to the
IP address specified in the ‘Gateway’ setting for routing
onwards to the specified network. This routing process may
occur several times as the data travels across different
networks enroute to its destination.
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