HP 3000 Manuals HP 3000 Manuals
Network Transport IP Subnetting Enhancement
by Bryan Wing
Business Networks Division
This article describes in detail the software enhancements that have been
made to the Network Transport to support IP subnetting.
Previous to this enhancement, IP network addresses for the 3000/XL links
consisted of a network address and a host address. With IP subnetting,
the host address space can be partitioned by assigning subnet addressing.
The result of this is a network address, a subnet address, and a host
address. This can be accomplished with Class A, Class B, and Class C IP
addressing, although the latter would leave limited host numbers.
The following example is of a Class B network address with a 6-bit wide
subnet field:
+---------------------------------------------------------------+
| | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | |
B |1 0| NETWORK | SUBNET | Host Address |
| | | |
+---------------------------------------------------------------+
octet 1 octet 2 octet 3 octet 4
Notice the three levels and that the subnet portion takes up 6 bits of
the Class B Host Number range. Unlike the Network field, the subnet
field is variable. This allows for flexibility in designing the overall
network. The larger number of nodes per subnet requires a smaller subnet
field. The larger number of subnets, the larger the subnet field.
The IP subnet field is specified using a mask. This mask will identify
those bits which are subnet bits. The IP subnet configuration is
accomplished via NMMGR. The screen NETXPORT.NI.<niname>.PROTOCOL.IP will
allow the addition of an IP subnet mask as an optional parameter. The
screen NETXPORT.NI.<niname>.INTERNET.<gatewayn> will allow the
configuration of the subnet of the reachable network. The IP Subnet Mask
fields in NMMGR will expect a decimal representation.
It is recommended that all subnets on the same network use the same
subnet mask. It is also recommended that the subnet mask is aligned
adjacent to the network number along byte boundaries. Remember, do not
assign an IP address where the network address and/or node address bits
are all off (all 0s) or all on (all 1s).
The following are some examples of IP subnets and masks:
1 1 1 1 1 1 1 1 1 1 2 2 2 2 2 2 2 2 2 2 3 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+---------------------------------------------------------------+
| | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | |
A |0| NETWORK | SUBNET | Local Address |
| | | | |
+---------------------------------------------------------------+
octet 1 octet 2 octet 3 octet 4
1 1 1 1 1 1 1 1 1 1 2 2 2 2 2 2 2 2 2 2 3 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+---------------------------------------------------------------+
| | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | |
Mask|1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1|0 0 0 0 0 0 0 0 0 0 0|
| | | | |
+---------------------------------------------------------------+
octet 1 octet 2 octet 3 octet 4
Mask = 255.255.248.00
1 1 1 1 1 1 1 1 1 1 2 2 2 2 2 2 2 2 2 2 3 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+---------------------------------------------------------------+
| | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | |
B |1 0| NETWORK | SUBNET | Local Address |
| | | | |
+---------------------------------------------------------------+
octet 1 octet 2 octet 3 octet 4
1 1 1 1 1 1 1 1 1 1 2 2 2 2 2 2 2 2 2 2 3 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+---------------------------------------------------------------+
| | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | |
Mask|1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1|0 0 0 0 0 0 0 0 0 0|
| | | | |
+---------------------------------------------------------------+
octet 1 octet 2 octet 3 octet 4
Mask = 255.255.252.000
1 1 1 1 1 1 1 1 1 1 2 2 2 2 2 2 2 2 2 2 3 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+---------------------------------------------------------------+
| | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | |
C |1 1 0| NETWORK |S N| Address |
| | | | |
+---------------------------------------------------------------+
octet 1 octet 2 octet 3 octet 4
1 1 1 1 1 1 1 1 1 1 2 2 2 2 2 2 2 2 2 2 3 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+---------------------------------------------------------------+
| | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | |
Mask|1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1|0 0 0 0 0 0|
| | | | |
+---------------------------------------------------------------+
octet 1 octet 2 octet 3 octet 4
Mask = 255.255.255.192
In the beginning the IP world consisted of independent networks linked
together. Not all networks had the same format for the IP address.
There were class A, B, and C addresses. The purpose of this division was
a better utilization of the space available in the IP address (32 bits).
The IP address is divided into Network number and Host number. If too
many bits are assigned for the network portion, fewer number of bits
remain available for the host portion. This allows for a small number of
nodes per network. The reverse allows for a small number of networks and
large number of nodes per network. Subnetting allows for a subdivision
of large networks.
![[]](../../pic3k/M000120/F09073c50.gif)
The example above depicts three networks: A, B, and C. Network A and C
have been divided into subnets. Network A has been broken into subnets
A1, A2, and A3. Network C has been broken into subnets C1 and C2. The
decision to subnet a network is up to the local network itself and does
not affect other networks. A network being subnetted is only relevant
within that network and does not affect nodes on other networks.
IP datagrams would be routed in the following manner:
* If node a11 wants to send an IP datagram to node c12, node a11 would
choose the route a11->a17->c19->c12.
* If node a11 wants to send an IP datagram to node b03, node a11 would
choose the route a11->a15->a33->a37->b03.