diff --git a/docs/ipam.md b/docs/ipam.md
index 706ad842..35bad67e 100644
--- a/docs/ipam.md
+++ b/docs/ipam.md
@@ -20,7 +20,7 @@ plugin) uses to request IP addresses.
 
 The commands supported by the allocator via the http interface are:
 
-- Allocate: request one IP address for a container
+- Allocate: request one IP address for a container in a subnet
 - Free: return an IP address that is currently allocated
 - Claim: request a specific IP address for a container (e.g. because
   it is already using that address)
@@ -46,18 +46,23 @@ freed.
 
 ### The Allocation Process
 
+Each allocation request either specifies a subnet to allocate within,
+or if no subnet is specified this is taken as a request to allocate in
+a pre-defined default subnet.
+
 When a peer owns some range(s), it can allocate freely from within
 those ranges to containers on the same machine. If it runs out of
-space (all owned ranges are full), it will ask another peer for space:
+space in a subnet (all owned ranges are full), it will ask another
+peer for space:
   - it picks a peer to ask at random, weighted by the amount of space
-    owned by each peer
+    owned by each peer in the subnet
     - if the target peer decides to give up space, it unicasts a message
       back to the asker with the newly-updated ring.
     - if the target peer has no space, it unicasts a message back to the
       asker with its current copy of the ring, on the basis that the
       requestor must have acted on out-of-date information.
   - it will continue to ask for space until it receives some, or its
-    copy of the ring tells it all peers are full.
+    copy of the ring tells it all peers are full in that subnet.
 
 ### Claiming an address
 
diff --git a/site/ipam.md b/site/ipam.md
index 836e980f..b29cdca4 100644
--- a/site/ipam.md
+++ b/site/ipam.md
@@ -9,10 +9,10 @@ Weave can automatically assign unique IP addresses to each container
 across the network. To make this work, weave must be told on startup
 what range of addresses to allocate from, for example:
 
-    host1# weave launch -iprange 10.2.3.0/24
+    host1# weave launch -iprange 10.2.0.0/16
 
-The `run`, `start`, `attach`, `expose` and `hide` commands will then
-fetch an address automatically if none is specified, i.e.:
+The `run`, `start`, `attach`, `detach`, `expose` and `hide` commands
+will then fetch an address automatically if none is specified, i.e.:
 
     host1# C=$(weave run -ti ubuntu)
 
@@ -22,7 +22,35 @@ You can see which address was allocated with
     host1# weave ps $C
     a7aff7249393 7a:51:d1:09:21:78 10.2.3.1/24
 
-The `-iprange` parameter is given in [CIDR
+You may wish to [run containers on different
+subnets](features.html#application-isolation). To set a default
+subnet, launch with `-ipsubnet`:
+
+    host1# weave launch -iprange 10.2.0.0/16 -ipsubnet 10.2.3.0/24
+
+`-iprange` should cover the entire range that you will ever use for
+allocation, and `-ipsubnet` is the subnet that will be used when
+you don't explicitly specify one.
+
+Then, to request the allocation of an address from a subnet other than
+the default, use `--subnet`:
+
+    host1# C=$(weave run --subnet 10.2.7.0/24 -ti ubuntu)
+
+To explicitly request the default subnet, use `--subnet default`.
+
+Note that `-iprange` can be smaller than `-ipsubnet`, if you want to
+use a mixture of automatically-allocated addresses and manually-chosen
+addresses, and have the containers communicate with each other. For
+example, if you say:
+
+    host1# weave launch -iprange 10.9.0.0/17 -ipsubnet 10.9.0.0/16
+
+then you can run all containers in the 10.9.0.0/16 subnet, with IPAM
+using the lower half and leaving the upper half free for manual
+allocation.
+
+The subnet parameters are written in [CIDR
 notation](http://en.wikipedia.org/wiki/Classless_Inter-Domain_Routing)
 format - in this example "/24" means the first 24 bits of the address
 form the network address and the allocator is to allocate container
@@ -117,8 +145,7 @@ reports on the current status of the weave router and IP allocator:
 ````
 weave router git-8f675f15c0b5
 ...
-Allocator subnet 10.2.1.0/24
-  Free IPs: ~98.0%, 62 local, ~189 remote
+Allocator universe 10.2.0.0/16 (default subnet 10.2.1.0/24)
 Owned Ranges:
   10.2.1.1 -> 96:e9:e2:2e:2d:bc (host1) (v3)
   10.2.1.128 -> ea:84:25:9b:31:2e (host2) (v3)