Subnetting is an optional
addressing scheme that allows you to partition the host address portion
of an internet address into discrete subnetworks. This allows you
to have multiple physical networks without requiring you to obtain
multiple network addresses. The physical networks are connected
via gateways.
For example, if you have a large installation with many interconnected
nodes, you could run into hardware configuration restrictions or
performance degradation if you tried to place all nodes on the same
physical network. With subnetting you can install several smaller
physical networks but have them all share the same network address.
When messages with subnet addresses are routed across the network,
the internet address is ANDed with the subnet mask to determine
the subnetwork address. (0 values in the subnet mask convert corresponding
bits in the IP address to 0.)
Figure 6-4 “Internet
Address 192.6.12.33 ANDed with Subnet Mask 255.255.255.224” shows how a Class
C Internet address and a subnet mask combine to form a subnet address.
When the internet address is ANDed with the subnet mask, the
zero values in the host portion of the subnet mask will “overwrite” the
corresponding bits of the host portion of the internet address and
the resultant subnet address will be 192.6.12.32 as shown in Figure 6-4 “Internet
Address 192.6.12.33 ANDed with Subnet Mask 255.255.255.224” above. Non-zero values in
the subnet mask indicate that the corresponding bits in the internet
address do not change. The subnet mask may be fixed-length or variable-length.
IP Subnet Mask
The subnet field (the portion of an IP address that identifies
the subnet beyond the network portion of the address) can be all
0’s or all 1’s, as described in RFC 1878.To disallow
subnet fields with all ones or all zeroes (conform to RFC 1122 behavior),
set the ndd parameter ip_check_subnet_addr to 1 in the nddconf file (/etc/rc.config.d/nddconf).
Selecting
a Subnet Addressing Scheme |
 |
In addition to fixed-length subnet addressing, HP-UX11i v2
systems support variable-length subnet addressing. The advantages
of using variable-length subnet addressing over fixed-length subnet
addressing include the following:
Allows the local administrator to
easily increase/decrease the size of individual subnets when variable-length
subnet addressing is used in conjunction with a non-contiguous numbering
system (and/or ranges of numbers for each subnet that are non-contiguous).
The mirror-image counting feature, which will be discussed
in the following section, also allows for more possibilities in
the numbers that can be assigned to individual subnets.
Reduces the amount of rework that network planners
will have to do on network design after the initial plan has been
completed.
Fixed-length subnets were easy to implement, but growth restrictions
often meant that it was necessary to invest more time whenever a
change was made to a subnet after it had been originally designed.
This new feature also makes it possible to have more than
one subnet mask on a network.
As described previously, an internet address can be represented
as four fields separated by a period, each of which represents 8
bits of the overall address.
The subnet address is based on the host
address portion of the internet address. The host address portion
subdivides into subnet number and host number fields to accommodate
a given number of subnetworks and a given number of nodes per subnetwork.
The size of the subnet number field is determined by the subnet
mask, which is explained later in this section. The subnet number
field must contain a minimum of one bit.
In the example below, the IP address, 192.6.12.33, has a subnet
number of 1.
Refer to Figure 6-5 “Subnet
Address and Subnet Number of Class C Internet Address 192.6.12.33” earlier
in this chapter for an illustration of how the subnet mask is ANDed with
the IP address to form the subnet number.
The following rules apply when choosing a subnet addressing
scheme and an internet address:
All subnets on the same network must
have the same network address.
If your system is attached to more than one physical
network, the subnet addresses of the interfaces on your system cannot
be the same.
Do not assign a host address where all the bits
of the host number are 0 or all the bits are 1.
You may choose a fixed-length subnet addressing scheme in
which one subnet mask will be used in all subnets in your network.
You may also choose a variable-length subnet addressing scheme in
which the subnet masks may be varied depending on the size of the
subnets you want.
If you wish to implement variable-length subnetting, first
read the conceptual information, “Assigning Variable-Length
Subnet Addresses” later in this chapter. Then refer to
the specific task-oriented instructions in the “Manually
Installing and Configuring HP-UX LAN” chapter earlier in
this manual. For additional information, refer to the following
online man pages: ifconfig(1M), netstat(1), ppl.remotes(4), routing(7), and route(1M). Detailed descriptions of fixed-length and variable-length
subnet addressing follow.
Fixed-Length
Subnet Addressing |
|
This section shows you how to apply a fixed-length subnet
addressing scheme on a class C network. The same steps may be applied
on a Class A or Class B network.
Assigning
a Fixed-Length Subnet Mask
Subnet addressing is implemented by specifying a 32-bit subnet
mask in the ifconfig command or the netconf file when a
LAN interface card is assigned an internet address. All nodes on
a subnet must have the same subnet mask. Under the fixed-length
subnet address scheme, all subnets on a network, must have the same
subnet mask.
The subnet mask is ANDed with the address attached to a message
coming across the network to determine if that message should be
routed to a node on the local network or ignored.
Before a subnet mask is applied, the first three octets of
a Class C IP address represent the network address. If the subnet
mask 255.255.255.224 is applied, the first three octets of the address
and the first three bits of the last octet indicate the subnet address
while the last five bits of the last octet indicate the host address.
This is shown in the following figure.
With this subnet mask, up to 8 subnets with 30 nodes per subnet
are possible. Table 6-5 “Fixed-Length Subnet Addressing (Subnet Mask 255.255.255.224)” below lists
the valid internet address ranges for nodes in a network with the
subnet mask 255.255.255.224.
Table 6-5 Fixed-Length Subnet Addressing (Subnet Mask 255.255.255.224)
Subnet Address
(dot
notation) | Internet
Address Range (dot notation) | Subnet Broadcast Address |
|---|
n.n.n.0 | n.n.n.1
- n.n.n.30 | n.n.n.31 |
n.n.n.32 | n.n.n.33
- n.n.n.62 | n.n.n.63 |
n.n.n.64 | n.n.n.65
- n.n.n.94 | n.n.n.95 |
n.n.n.96 | n.n.n.97
- n.n.n.126 | n.n.n.127 |
n.n.n.128 | n.n.n.129
- n.n.n.158 | n.n.n.159 |
n.n.n.160 | n.n.n.161
- n.n.n.190 | n.n.n.191 |
n.n.n.192 | n.n.n.193
- n.n.n.222 | n.n.n.223 |
n.n.n.224 | n.n.n.225 - n.n.n.254 | n.n.n.255 |
Example
of Subnets with a Fixed-Length Subnet Mask
The following example
shows four subnetworks within the 192.6.12 network along with the netconf entries necessary to configure these subnetworks.
The complete network map is shown in Figure 6-8 “Network
Map I”.
Summary network information:
Company division network = 192.6.12
Subnet mask: 255.255.255.224 |
Facility LAN subnet number = 1
Host address range: 22 to 62
Host A internet address: 192.6.12.33 for network interface lan0
Host B internet address: 192.6.12.36 for network interface lan0
Host C internet address: 192.6.12.34 for network interface lan0
Host D internet address: 192.6.12.35 for network interface lan0 |
Marketing Department subnet number = 2
Host address range: 65 to 94
Host A internet address: 192.6.12.65 for network interface lan1 |
R & D Department subnet number = 3
Host address range: 97 to 126
Host A internet address: 192.6.12.97 for network interface lan1:0
Host B internet address: 192.6.12.98 for network interface lan1:1 |
Manufacturing Department subnet number = 4
Host address range: 129 to 158
Host B internet address: 192.6.12.130 for network interface lan1 |
Configuring
Hosts on Fixed-Length Subnets Using the netconf file
There are two ways to configure a subnet mask:
Use SAM to configure the subnet mask.
Use the ifconfig command. These changes will disappear, however,
when the system reboots.
To set a subnet mask, you may either include it in the SUBNET_MASK
variable in the netconf file or the ifconfig command. The netconf file contains information you entered in SAM and
this information is used to start networking when the system reboots.
If you configure your network interfaces using SAM, SAM will enter
the proper information in the netconf file for you. The examples below are netconf examples for the hosts in the example above after
you have configured them in SAM:
Host A:
INTERFACE_NAME[0]=”lan0”
IP_ADDRESS[0]=”192.6.12.33”
SUBNET_MASK[0]=”255.255.255.224” |
INTERFACE_NAME[1]=”lan1”
IP_ADDRESS[1]=”192.6.12.65”
SUBNET_MASK[1]=”255.255.255.224” |
Host B:
INTERFACE_NAME[0]=”lan0”
IP_ADDRESS[0]=”192.6.12.36”
SUBNET_MASK[0]=”255.255.255.224” |
INTERFACE_NAME[1]=”lan1”
IP_ADDRESS[1]=”192.6.12.130”
SUBNET_MASK[1]=”255.255.255.224” |
Host C:
INTERFACE_NAME[0]=”lan0”
IP_ADDRESS[0]=”192.6.12.34”
SUBNET_MASK[0]=”255.255.255.224” |
INTERFACE_NAME[1]=”lan0:1”
IP_ADDRESS[1]=”192.6.12.37”
SUBNET_MASK[1]=”255.255.255.224” |
INTERFACE_NAME[2]=”lan1”
IP_ADDRESS[2]=”192.6.12.97”
SUBNET_MASK[2]=”255.255.255.224” |
INTERFACE_NAME[3]=”lan1:1”
IP_ADDRESS[3]=”192.6.12.98”
SUBNET_MASK[3]=”255.255.255.224” |
Host D:
INTERFACE_NAME[0]=”lan0”
IP_ADDRESS[0]=”192.6.12.35”
SUBNET_MASK[0]=”255.255.255.224” |
INTERFACE_NAME[1]=”lan1”
IP_ADDRESS[1]=”192.6.20.1”
SUBNET_MASK[1]= 255.255.255.0 |
In addition, every other host on each subnetwork would require
the subnet mask 255.255.255.224 in their netconf file entries.
Example
of Network Map with Fixed-Length Subnets
This sample network combines networks, subnets with a fixed-length
mask, and clusters previously described and illustrated in this
chapter along with a sample worksheet that provides configuration
information necessary to attach these systems to the networks.
Table 6-6 Network Map I Worksheet
Host | Inter-
face
Alias | Internet Address | Station Address | Cnode Type |
|---|
A | mkt_32
mkt_64 | 192.6.12.33 192.6.12.65 | 08000909030D 080009080102 | Server |
A1 | mkt_a1 | 192.6.12.67 | 080009005201 | Client
1 |
A2 | mkt_a2 | 192.6.12.66 | 080009003001 | Client
2 |
B | mfg_32
mfg_128 | 192.6.12.36 192.6.12.130 | 080009005201 080009000C24 | N/A |
B1 | mfg__b1
b1_slip | 192.6.12.131 192.1.1.2 | 080009001001 N/A | N/A |
B2 | mfg_b2 | 192.6.12.132 | 080009002125 | N/A |
C | rd_32
rd_36
rd_96
rd_99 | 192.6.12.34
192.6.12.37
192.6.12.97
192.6.12.98 | 080009267C14080009267C14 080009260C85
080009260C85 | Server |
C1 | rd_c1 | 192.6.12.99 | 08000900079C | Client
1 |
C2 | rd_c2 | 192.6.12.98 | 08000900601A | Client
2 |
D | div1_32
div_gw | 192.6.12.35 192.6.20.1 | 080009000740 080009000B30 | N/A |
Div2 | div2_128
div2_gw | 192.6.14.129 192.6.20.2 | 080009006041 080009007104 | N/A |
Div3 | div3_128
div3_gw | 192.6.13.130 192.6.20.3 | 080009004020 080009010312 | N/A |
E
(SLIP) | e_slip | 192.1.1.1 | N/A | N/A |
Subnet mask = 255.255.255.224
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