User's Manual

Configuration Guide of XGS-5240-Series
194
VLAN IP Subnet
Gateway
Address
Usable
Hosts
Customer
Hosts
Needed
Hosts
21 1.1.1.0/28 1.1.1.1 14 13 10
22 1.1.1.16/29 1.1.1.17 6 5 5
23 1.1.1.24/30 1.1.1.25 2 1 1
In above table, the needed hosts may be 10 in VLAN 21 and assign a subnet with mask of 28
bits—1.1.1.0/28. However, subnet 1.1.1.0 of network segment, subnet broadcast address
1.1.1.15 and the default gateway address 1.1.1.1 can not become the host address, address
range within 1.1.1.2 to 1.1.1.14 can become the host address. So the usable host addresses
total is 13 (2
32-28
-3=13), however, only 10 addresses can satisfy the requirement for VLAN21.
The rest may be deduced by analogy, the needed host addresses is 5 for VLAN 22. To
satisfy request, it needs to assign a subnet with mask of 29 bits (1.1.1.16/29) for VLAN 22. The
needed host address is only 1 for VLAN 23, but it occupies subnet 1.1.1.24/30.
The needed addresses is 16 (10+5+1) for three VLANs, but the excellent method also
occupies 28 addresses (2
32-28
+2
32-29
+2
32-30
=28) according to address division of generic VLAN,
so waste many addresses. Besides, if the needed hosts only reach to 3, but not to 10 hosts,
and a subnet with mask of 28 bits is assigned to VLAN 21 before, here, the redundant
addresses are wasted because they can not be used by other VLAN.
The above division is difficult to network update. If client of VLAN23 needs to add 2 hosts
and client does not want to change the assigned IP address, but the addresses after 1.1.1.24
are assigned to others, so we should assign a subnet with mask of 29 bits and a new VLAN to
this client. Here, there are 3 hosts for client of VLAN23 only, but this client is assigned two
subnets (they are not in the same VLAN), therefore, it is difficult to management.
We can see that the number of the wasting IP address (such as subnet number, broadcast
address, default gateway address) is considerable and badly reduce the addressing flexility
that waste many addresses. Therefore, Super VLAN is developed for solving the problem.
Super VLAN advantages are shown in the following:
Reduce IP address number about subnet number, default gateway address and broadcast
address
Implement that the different broadcast domains use addresses of the same subnet segment
Enhance addressing flexility
Reduce the address waste