Subnetting a 38 Network Address

The most important thing to remember when using a 24 bit subnet of our Class A 38 network is that it is *NOT* a real Class C network. Due to this fact it is easier to subnet than a Class C.

Remember that the subnet portion of an IP address cannot be all 0s or all 1's. Thus, when a Class C network is subnetted you cannot use a 1 bit subnet mask, since the subnet portion will either be 1 or 0 (which is all 1s and all 0s respectively). If you use a 2 bit subnet mask, you lose the 00 and 11 subnet and only the 01 and 10 subnets are usable.

The network portion of a Class A address is only 8 bits; the following 16 bits are part of the subnet portion of the address. If you add an additional bit to the subnet mask, then the number of subnet bits is 17 not 1. The subnet portion of the address is not 0 or 1 but xxxxxxxx xxxxxxxx 0 or xxxxxxx xxxxxxxx 1. This also follows for any number of subnet bits that are added. The only addressing space that is lost is the all 0s and all 1s hosts as these are still reserved for the network address, and broadcast address.

The following are the numbering schemes for the most common Subnet configurations for subnetting a 38.x.y.0 address:

25 Bit

Subnetwork Bits:         25
Number of Subnets:        2
Hosts per Subnet:       126
Total Number of Hosts:  252
Netmask:                255.255.255.128

Subnet 
Bits  Network  Range  Valid Hosts
------  -------  -----  -----------
0     0    0-127  1-126
1   128  128-255  129-254

26 Bit

Subnetwork Bits:         26
Number of Subnets:        4
Hosts per Subnet:        62
Total Number of Hosts:  248
Netmask:                255.255.255.192

Subnet 
Bits  Network  Range  Valid Hosts
------  -------  -----  -----------
0 0    0   0-63  1-62
0 1   64  64-127  65-126
1 0  128  128-191  129-190
1 1  192   192-255  193-254

27 Bit

Subnetwork Bits:  27
Number of Subnets:   8
Hosts per Subnet:  30
Total Number of Hosts: 240
Netmask:  255.255.255.224

Subnet 
Bits     Network         Range           Valid Hosts
------   -------         -----           -----------
0 0 0    0   0-31  1-30
0 0 1    32  32-63  33-62
0 1 0    64  64-95  65-94
0 1 1    96  96-127  97-126
1 0 0  128  128-159  129-158
1 0 1  160  160-191  161-190
1 1 0  192  192-223  193-222
1 1 1  224   224-255  225-254

28 Bit

Subnetwork Bits:         28
Number of Subnets:       16
Hosts per Subnet:        14
Total Number of Hosts:  224
Netmask:                255.255.255.240

Subnet
Bits     Network         Range           Valid Hosts
------   -------         -----           -----------
0 0 0 0    0   0-15  1-14
0 0 0 1   16  16-31  17-30
0 0 1 0   32  32-47  33-46
0 0 1 1   48  48-63  49-62
0 1 0 0   64  64-79  65-78
0 1 0 1   80  80-95  81-94
0 1 1 0   96  96-111  97-110
0 1 1 1  112  112-127  113-126
1 0 0 0  128  128-143  129-142
1 0 0 1  144  144-159  145-158
1 0 1 0  160  160-175  161-174
1 0 1 1  176  176-191  177-190
1 1 0 0  192  192-207  193-206
1 1 0 1  208  208-223  209-222
1 1 1 0  224  224-239  225-238
1 1 1 1  240   240-255  241-254

29 Bit

Subnetwork Bits:         29
Number of Subnets:       32
Hosts per Subnet:         6
Total Number of Hosts:  192
Netmask:                255.255.255.248

30 Bit

Subnetwork Bits:         30
Number of Subnets:       64
Hosts per Subnet:         2
Total Number of Hosts:  128
Netmask:                255.255.255.252

Subnetting - What, When and How

What is Subnetting?

Subnetting is essentially the modification of a single IP network to create two or more logically visible sub-sections. It entails changing the subnet mask of the local network number to produce an even number of smaller network numbers, each with a corresponding range of IP addresses.

When is subnetting necessary?

Subnetting is required when one network number needs to be distributed across multiple LAN segments. This may be the case in instances when:

• A company uses two or more types of LAN technology (for example, Ethernet, Token Ring) on their network.
  
  
• Two network segments are restricted by distance limitations (for example, remote offices linked via point-to-point circuit).
  
  
• Segments need to be localized for network management reasons (accounting segment, sales segment, etc.).
  
  
• Hosts which dominate most of the LAN bandwidth need to be isolated.

Advantages and Disadvantages

Hopefully your decision to subnet happened before your workstations were assigned IP addresses. This will allow for much more flexibility in your segment layout. Subnetting can provide you with easier network management capabilities and also lends itself to faster troubleshooting. Additionally, subnetting keeps the size of the Internet's routing tables down, since you won't be adding an additional network number for each segment.

The routing announcement of your network to the outside world will still be based on your single network number. Keep in mind, though, that subnetting will decrease the total amount of IP addresses available to you and may require purchasing additional hardware such as a router. This will be covered later.

How It Works

An IP address is a 32 bit number divided into 4 sections of 8 bits called octets. Each octet is usually converted from binary to decimal form and separated with a dot to make it readable. The address can also be split into a network and host portion.

The network portion always remains fixed for a particular network, while the remaining bits which make up the host portion can be altered to give the range of addresses to assign to hosts.

In order to determine where the network portion ends and the host portion begins, a subnet mask (or netmask, or just mask) is used to fix the network portion and allow the host portion to be changed. As an example, we will use the IP number 38.9.211.0 with a subnet mask of 255.255.255.0. Changing to binary, we get:

A binary 1 will "mask" the bit, and a 0 will deem it variable. So, for this case, we refer to the netmask as 24 bits, or /24.

38.9.211.0/24 

The fixed bits become the network portion and the remaining bits become the host portion, so in this case there are 8 host bits which account for a range from 0-255. IP protocol standards dictate that we use addresses of all 0's to refer to the network as a whole, and addresses of all 1's to refer to the broadcast of all hosts on the network, so these host addresses may not be used. This limits our host range from 1 - 254.

38.9.211.0/24	the entire /24 network
38.9.211.1 - 38.9.211.254	range of valid hosts
38.9.211.255	broadcast address of all hosts on the network

Once you receive your network number, whether it's a /24, /25, or /27, you have the ability to extend the network portion further into the host-number field by lengthening the netmask. The number of bits that you extend into the original host portion determines how many segments, or subnets, you will produce.

To be specific, lengthening the mask by n bits will produce 2^n subnets.

Let's illustrate what would happen if we used 2 subnet bits. The host portion would be reduced to six bits. There are 4 different combinations of arranging the 2 subnet bits, so we arrive at 4 unique subnets.

Subnet 1	00000000 - 00111111	(.0 - .63)
Subnet 2	01000000 - 01111111	(.64 - .127)
Subnet 3	10000000 - 10111111	(.128 - .191)
Subnet 4	11000000 - 11111111	(.192 - .255)

What we have accomplished is creating 4 network numbers from 1. Once again, adhering to our rule that all 0's is a network, and all 1's is a broadcast, we can see where each subnetwork begins and ends.

Network	Host Ranges	Broadcast
38.9.211.0/26 **	38.9.211.1 - 38.9.211.62	38.9.211.63
38.9.211.64/26	38.9.211.65 - 38.9.211.126	38.9.211.127
38.9.211.128/26	38.9.211.129 - 38.9.211.190	38.9.211.191
38.9.211.192/26 **	38.9.211.193 - 38.9.211.254	38.9.211.255

**An Important Note

Some older routers do not send subnet masks for every announced route. These routers do not understand the all 0's and all 1's subnets, so they may not be used. However, if you are using an assigned number from our net 38 block (for example, 38.x.x.x), or if your router allows you to provide netmasks in its routing table, these subnets are routable

Sample Subnetting Topology

The figure above illustrates a topology which can route the same network number over 3 segments, one of which is linked across a PPP connection. The /24 network was split into 4 subnets (since you may only produce 2^n subnets) using a /26 mask.

Configuration

Each router and workstation will use a 255.255.255.192 subnet mask in its IP configuration. The default gateway will always be the IP address of the closest router to the Internet on the segment.

TCP/IP Config

Subnet 3 - 38.9.211.128/26
The following use 38.9.211.129 as a default gateway:

38.9.211.130
28.9.211.131
38.9.211.132

Subnet 2 - 38.9.211.64/26
The following use 38.9.211.65 as a default gateway:

38.9.211.69
38.9.211.68
38.9.211.67
38.9.211.66

Subnet 1 - 38.9.211.0/26
The following use 38.9.211.1 as a default gateway:

38.9.211.5
38.9.211.4
38.9.211.3
38.9.211.2

Static vs. Dynamic Routing

Internal routing may be accomplished statically or dynamically. For dynamic routing, an internal protocol such as rip may be used. Each router will create its own routing table based on rip announcements. If static routing is preferred, the tables will need to be created manually. The table below illustrates what the routing table will look like on each router. The default gateway is identified by the 0.0.0.0 route which refers to any route that is not explicitly matched in the routing table.

Router 4 (38.9.211.129)
0.0.0.0 via PPP WAN link

Router 3 (38.9.211.69)
0.0.0.0 via 38.9.211.65
38.9.211.128 255.255.255.192 via PPP WAN link

Router 2 (38.9.211.5)
0.0.0.0 via 38.9.211.1
38.9.211.128 255.255.255.192 via 38.9.211.69
38.9.211.64 255.255.255.192 via 38.9.211.65

Router 1 (38.9.211.1)
0.0.0.0 via WAN link or PSINet POP IP address
38.9.211.128 255.255.255.192 via 38.9.211.5
38.9.211.64 255.255.255.192 via 38.9.211.5
38.9.211.0 255.255.255.0 via 38.9.211.1 (Full /24 announcement)