RFC 3442 / START HERE
DHCP classless routes, before the details
DHCPv4 Classless Static Route Option. DHCP means Dynamic Host Configuration Protocol; classless means each route carries its own network mask.
A DHCP server can tell a client which router to use for particular IPv4 destination networks. Option 121 packages these routes with explicit mask widths, including a default route when needed.
The idea in one picture
Server supplies routes
Client installs a table
Next hop forwards packets
A simplified picture of the idea; the chapters explain the actual mechanisms.
Why it exists
Older DHCP static routes assumed a mask from the address. RFC 3442 replaces that option with explicit classless routes, while defining how supporting clients handle older alternatives. It updates RFC 2132 rather than replacing DHCPv4.
A familiar example
A fictional laptop receives a route to a private lab network through one router and a default route through another. The laptop installs the supplied networks and next hops. Correct route encoding alone does not prove the server is trustworthy.
Helpful background
You can start here without reading another RFC. If these ideas are unfamiliar, these short introductions may help.
What you’ll learn next
Begin with chapter one, or choose a topic.
- Send routes with their masksDHCPv4 can give a client routes to particular IPv4 networks, each with an explicit mask and a next-hop router.→
- Decode a partial final octetA mask width need not be a multiple of eight. Read enough destination octets, then remove every host bit.→
- Request routes and apply precedenceSupporting clients request option 121 ahead of older alternatives. If 121 arrives, it controls route installation.→
- Reach another subnet on the same linkOne link can carry several IP subnets. A zero router address can represent direct delivery to another subnet on that same link.→
- Carry long route lists safelyA route list can outgrow one DHCP option instance or a small message. Concatenation and advertised message size address different limits.→
- Separate routing syntax from trustA route can be well formed and still send traffic to the wrong place. Option 121 does not authenticate the server or protect the next hop.→
This introduction explains RFC 3442 (2002). Read the original RFC ↗