So we're going to round out the theory part of our DHCPv4
discussion and talk about DHCPv4 options.
DHCP options are basically used to pass configuration information,
and even though they're called options, many of them are actually required.
We'll talk a little bit about those on the next slide.
Every DHCP implementation, whether you're running on Microsoft,
Cisco, Juniper, Infoblox, etc., uses DHCP options,
or supports DHCP options.
Many of these DHCP options are assigned by the Internet Engineering Task Force,
or the IETF, and if you're interested in looking at some of those options,
you can look at RFC 2132, which defines most of the commonly-used options.
Vendors can even use their own special option numbers.
There are some unassigned numbers that are very high in the option range,
and a lot of your vendors for wireless access points or IP phones,
will actually use some of those unused option numbers.
And DHCP options is exactly like you think it is.
It's option X number, and it has a value of Y,
and this will make a little more sense when we look at it in
the packet tracer here in just a little bit,
but basically you as an administrator can go into the DHCP server and
set some of the options that work on your network,
and those options are distributed to your clients.
Now some of the more common DHCP options that you'll see on production networks,
option 53, which is the DHCP message type,
and those are the DHCP messages that we discussed back when
we talked about the four-packet flow.
Those are the DHCPDISCOVER and DHCPOFFER, etc., etc.
There are 18 different values that you can use here.
Again,
we're not going to go into every single one of them because a lot
of them you'll only see in error situations,
and we'll cover some of them when we talk about
troubleshooting later on in the course.
But again,
you can go look at RFC 2132 and some of the other DHCP RFCs if
you really have trouble going to sleep at night and need
something to help along with that.
Another common DHCP option is option 6, or the domain server,
or the DNS server as it's known to nearly everyone else.
Again, this is not a required option,
but it's one of the whole points of having DHCP is to push
out a DNS server and a default route, and so on and so forth.
Another option that's used is the subnet mask,
and again,
this is one of those options that is technically called
an option in the DHCP implementation,
but it's kind of hard to have a DHCP implementation without a subnet mask.
I suppose you could use the classful subnet mask,
but I don't know of many networks that run a 10.
0.
0/8 network.
If you are, again, more power to you if that works for you,
but most networks will use a DHCP server that has a subnet mask
so that you only use as many IP addresses as makes sense for
the particular subnet you're on.
Option 51 is the lease time.
Again, for the dynamic type of address allocation,
you will have to have a lease time so that the client
knows when to stop requesting addresses,
or when to go request a new address, or the same address.
Option 3 is the router, or the default route.
Again,
one of the whole points of having DHCP is to pass
out the default route information.
And while it is an option,
again it's pretty much required by every DHCP server implementation.
And finally, option 150.
You know me as a Cisco voice guy, I'm not going to leave off option 150.
Option 150 is a standard option that's called TFTP server,
but it is most commonly used on Cisco voice networks so that
your Cisco phones know where to register themselves with to get
their configuration information.
The TFTP server, or option 150,
is where you put in the IP addresses of your publisher and
subscriber servers for your voice infrastructure.
Again, we definitely are not going to get into that here in this course, but option 150 if you're working on Cisco networks is actually pretty common.