PPP · Topic 3 of 3
IP Pools for PPP Clients
Learn how RouterOS IP pools hand out address ranges to PPP, DHCP and HotSpot clients, how ranges and next pool work, and how to see used addresses.
- 4 min
- A pool is a named list of address ranges under
/ip pool. - A PPP profile uses a pool name in
remote-addressorlocal-address. - Each connecting client gets the next free address, and it is returned on disconnect.
What it is
An IP pool is a named range of addresses that RouterOS gives out to clients on request.
Pools are shared by several services, and PPP profiles, DHCP servers and HotSpot all draw from them. A pool holds no policy of its own.
It only keeps the list of addresses and tracks which ones are taken.
The pool is created under /ip pool, and a service refers to it by name.
How it works
A service asks the pool for an address, and the pool returns the next free one.
Creating a pool
/ip pool add name=vpn-pool ranges=10.10.10.10-10.10.10.50
/ip pool printThe ranges value is one or more address ranges. Separate several ranges with commas. A range can be written from one address to another, and a single address is also valid.
Using a pool in PPP
A pool name replaces an IP address in the remote-address or local-address setting.
/ppp profile add name=staff local-address=10.10.10.1 remote-address=vpn-poolEvery client that connects with this profile receives the next free address from vpn-pool. When the client disconnects, the address returns to the pool.
The router's own address, 10.10.10.1 above, should sit outside the pool range. Otherwise the pool may hand it to a client.
Fixed addresses and pools
A remote-address in the secret is an exact address, and it overrides the profile. Keep fixed addresses outside the pool range, so the pool never offers an address that a secret has reserved.
Point to point links
A PPP link is point to point. Each client gets its own address from the pool, and the dynamic interface is paired with the router address. Clients do not need to share a subnet in the usual sense.
The pool can be bigger than the number of expected clients. A pool with a given number of addresses allows at most that many simultaneous sessions with that profile.
Sizing a pool
Size the pool for the busiest moment, not for the average. If fifty staff may connect at the same time, the pool needs at least fifty free addresses, plus spare room for reconnects.
A client that drops without a clean logout may hold its address until the router notices that the link is gone. During that time the address is not free for a new client.
Next pool
The next-pool setting chains pools. When the first pool has no free address, the router takes one from the pool named in next-pool.
/ip pool add name=vpn-pool2 ranges=10.10.20.10-10.10.20.50
/ip pool set vpn-pool next-pool=vpn-pool2This extends capacity without enlarging the first range.
Checking what is in use
/ip pool used print/ip pool used print lists each address that is in use and the pool and owner that hold it. It is the first command to run when clients report that they cannot connect, since an empty pool is a common cause.
Pools are not only for PPP
The same pool can serve a DHCP server through its address-pool setting, and it can serve HotSpot users through the HotSpot server. Use separate pools for each service so that ranges never overlap.
What to remember for the exam
Six facts cover most pool questions.
- A pool is a named list of address ranges under
/ip pool. - A PPP profile uses a pool name in
remote-addressorlocal-address. - Each connecting client gets the next free address, and it is returned on disconnect.
- Several ranges in one pool are separated by commas.
next-pooltakes over when the first pool is empty./ip pool used printshows which addresses are taken.
Keep the router's own tunnel address and any fixed secret addresses outside the pool range.
Common mistakes
These four errors appear again and again.
Putting the router address inside the pool. The pool may then hand the router address to a client, which causes an address clash.
Making the pool too small. When the pool runs out, new clients fail to connect while existing ones keep working.
Overlapping pools between services. A DHCP pool and a PPP pool that share addresses cause duplicates. Give each service its own range.
Typing a pool name that does not exist. The profile then has no valid address source, and clients cannot get an address.
Changing a pool range does not move clients that already hold an address. They keep their old address until they reconnect.
Check yourself
Work through these three questions.
How does a PPP profile use an IP pool?
Put the pool name in
remote-addressorlocal-address. Each connecting client then gets the next free address from the pool.What does
next-pooldo?It names a second pool that the router uses when the first pool has no free address.
Which command shows the addresses that are currently in use?
/ip pool used printlists the addresses in use and who holds them.
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