Before even requesting a subnet change, is a good idea to consider how you expect your IP address needs to change for the forseeable future. For instance a college may have plans to wire up more student rooms, construct a new building or to expand central computing facilities. Departments may be expecting such things as new buildings, expansion in terms of number of students or staff or the introduction of new computing clusters. All of these will create the demand for more IP address space.
With the use of technologies such as VLANs (virtual LANs), it is now possible for annexe networks to be integrated with the network on the main site, despite considerable physical separation. In some cases (especially on networks which are currently shared between colleges or departments) this will require the purchase of additional switching hardware, for which there will be a charge. The use of VLANs can therefore mean that while the subnet used for a remote annexe can be reclaimed, the main network reaches full capacity and requires expansion.
Further planning will be required as to how to distribute IP addresses within the new IP address block: many IT officers like to maintain separate ranges for servers, DHCP-assigned addresses, static addresses, network hardware (hubs & switches), and so forth. If parts of a subnet have been sub-let to other institutions, you must bear them in mind as the change progresses and ensure that they are kept informed of what is happening.
If you operate a firewall within your unit, the firewall rules will need to be amended so that traffic to and from both the old and new subnets can pass through as necessary while the subnet change is in place. Additional configuration changes may be required to access restrictions applying to various servers, for instance tcp_wrappers or departmental intranet web pages.
Routeing firewalls may present an additional problem, in that not all can cope with multiple subnets on the inside. With these, the only option may be to go for a "big-bang" style changeover to the new subnet. Bridging firewalls should in general not suffer from such problems.
Please consider how you will get the work done within the agreed timescale. There may be obstacles such as systems which cannot be reconfigured at certain times (for instance during exams or admissions), or systems for which an IP address change is non-trivial. Please take these into account in your plans and provisionally schedule the necessary work. Many subnet migrations have overrun because critical systems have been left until last and then cannot immediately be changed.
Once you have decided upon your needs, discussed and agreed them with email@example.com, it is time to start the procedure. Hostmaster will set up IP routing for the new IP network range and amend the DNS web interface to include your new IP range.