diff --git a/CHANGELOG.md b/CHANGELOG.md
index 155cb7a..7baeb3a 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -2,13 +2,23 @@
 
 ## Next
 
+### Removed
+
+ - NixOS 20.09 support. Its docker-compose does not support the
+   `networks.<name>.name` option, which is important in later versions.
+
 ### Changed
 
 * Healthcheck-based dependencies in `service.depends_on`.
+* The `project.name` option is now mandatory.
 
 ### Added
 
 * Support `service.healthcheck` for defining custom healthchecks.
+* Arion now declares a `networks.default` by default, with `name` set to
+  `project.name`. This improves compatibility with container runtimes by
+  copying pre-existing behavior. Most users will want to keep using this
+  behavior, but it can be disabled with `enableDefaultNetwork`.
 
 ## 0.1.3.0 -- 2020-05-03
 
diff --git a/arion-compose.cabal b/arion-compose.cabal
index 3aa58b0..6b0c14b 100644
--- a/arion-compose.cabal
+++ b/arion-compose.cabal
@@ -17,6 +17,7 @@ extra-source-files:  CHANGELOG.md, README.asciidoc,
                      src/haskell/testdata/**/*.json
 data-files:          nix/*.nix
                    , nix/modules/composition/*.nix
+                   , nix/modules/networks/*.nix
                    , nix/modules/nixos/*.nix
                    , nix/modules/service/*.nix
                    , nix/modules/lib/*.nix
diff --git a/docs/modules/ROOT/partials/NixOSOptions.adoc b/docs/modules/ROOT/partials/NixOSOptions.adoc
index 690b23b..131bf4f 100644
--- a/docs/modules/ROOT/partials/NixOSOptions.adoc
+++ b/docs/modules/ROOT/partials/NixOSOptions.adoc
@@ -22,6 +22,30 @@ Attribute set that will be turned into the docker-compose.yaml file, using Nix's
 Type:: attribute set
 No Default:: {blank}
 
+No Example:: {blank}
+
+== enableDefaultNetwork
+
+Whether to define the default network:
+
+```nix
+networks.default = {
+  name = config.project.name;
+};
+```
+
+
+[discrete]
+=== details
+
+Type:: boolean
+Default::
++
+----
+true
+----
+
+
 No Example:: {blank}
 
 == host.nixStorePrefix
@@ -70,6 +94,164 @@ No Default:: {blank}
 
 No Example:: {blank}
 
+== networks
+
+See link:https://docs.docker.com/compose/compose-file/#networks-top-level-element[Docker Compose#networks-top-level-element]
+
+
+[discrete]
+=== details
+
+Type:: lazy attribute set of submodules
+No Default:: {blank}
+
+No Example:: {blank}
+
+== networks.<name>.attachable
+
+See link:https://docs.docker.com/compose/compose-file/#attachable[Docker Compose#attachable]
+
+
+[discrete]
+=== details
+
+Type:: boolean
+No Default:: {blank}
+
+Example::
++
+----
+true
+----
+
+
+== networks.<name>.driver
+
+`"none"`, `"host"`, or a platform-specific value.
+See link:https://docs.docker.com/compose/compose-file/#driver[Docker Compose#driver]
+
+
+[discrete]
+=== details
+
+Type:: string
+No Default:: {blank}
+
+No Example:: {blank}
+
+== networks.<name>.driver_opts
+
+See link:https://docs.docker.com/compose/compose-file/#driver_opts[Docker Compose#driver_opts]
+
+
+[discrete]
+=== details
+
+Type:: lazy attribute set of raw values
+No Default:: {blank}
+
+No Example:: {blank}
+
+== networks.<name>.enable_ipv6
+
+Whether we've entered the 21st century yet.
+
+See link:https://docs.docker.com/compose/compose-file/#enable_ipv6[Docker Compose#enable_ipv6]
+
+
+[discrete]
+=== details
+
+Type:: boolean
+No Default:: {blank}
+
+No Example:: {blank}
+
+== networks.<name>.external
+
+When `true`, don't create or destroy the network, but assume that it
+exists.
+
+See link:https://docs.docker.com/compose/compose-file/#external[Docker Compose#external]
+
+
+[discrete]
+=== details
+
+Type:: boolean
+No Default:: {blank}
+
+No Example:: {blank}
+
+== networks.<name>.internal
+
+Achieves "external isolation".
+
+See link:https://docs.docker.com/compose/compose-file/#internal[Docker Compose#internal]
+
+
+[discrete]
+=== details
+
+Type:: boolean
+Default::
++
+----
+false
+----
+
+
+No Example:: {blank}
+
+== networks.<name>.ipam
+
+Manage IP addresses.
+
+See link:https://docs.docker.com/compose/compose-file/#ipam[Docker Compose#ipam]
+
+
+[discrete]
+=== details
+
+Type:: raw value
+No Default:: {blank}
+
+No Example:: {blank}
+
+== networks.<name>.labels
+
+Metadata.
+
+See link:https://docs.docker.com/compose/compose-file/#labels[Docker Compose#labels]
+
+
+[discrete]
+=== details
+
+Type:: attribute set of strings
+No Default:: {blank}
+
+No Example:: {blank}
+
+== networks.<name>.name
+
+Set a custom name for the network.
+
+It shares a namespace with other projects' networks. `name` is used as-is.
+
+Note the `default` network's default `name` is set to `project.name` by Arion.
+
+See link:https://docs.docker.com/compose/compose-file/#name[Docker Compose#name]
+
+
+[discrete]
+=== details
+
+Type:: string
+No Default:: {blank}
+
+No Example:: {blank}
+
 == out.dockerComposeYaml
 
 A derivation that produces a docker-compose.yaml file for this composition.
@@ -112,17 +294,14 @@ Name of the project.
 
 See link:https://docs.docker.com/compose/reference/envvars/#compose_project_name[COMPOSE_PROJECT_NAME]
 
+This is not optional, because getting the project name from a directory name tends to produce different results for different repo checkout location names.
+
 
 [discrete]
 === details
 
-Type:: null or string
-Default::
-+
-----
-null
-----
-
+Type:: string
+No Default:: {blank}
 
 No Example:: {blank}
 
diff --git a/examples/full-nixos/arion-compose.nix b/examples/full-nixos/arion-compose.nix
index 1524e3a..94890db 100644
--- a/examples/full-nixos/arion-compose.nix
+++ b/examples/full-nixos/arion-compose.nix
@@ -1,4 +1,5 @@
 {
+  project.name = "full-nixos";
   services.webserver = { pkgs, lib, ... }: {
     nixos.useSystemd = true;
     nixos.configuration.boot.tmpOnTmpfs = true;
diff --git a/examples/nixos-unit/arion-compose.nix b/examples/nixos-unit/arion-compose.nix
index 1edd4e5..642a18e 100644
--- a/examples/nixos-unit/arion-compose.nix
+++ b/examples/nixos-unit/arion-compose.nix
@@ -17,6 +17,7 @@
  */
 
 {
+  project.name = "nixos-unit";
   services.webserver = { config, pkgs, ... }: {
 
     nixos.configuration = {config, lib, options, pkgs, ...}: {
diff --git a/examples/traefik/arion-compose.nix b/examples/traefik/arion-compose.nix
index b6c6f7d..c43627f 100644
--- a/examples/traefik/arion-compose.nix
+++ b/examples/traefik/arion-compose.nix
@@ -9,7 +9,7 @@
 
  */
 { lib, pkgs, ... }: {
-
+  config.project.name = "traefik";
   config.services = {
     traefik = {
       image.command = [
diff --git a/nix/ci.nix b/nix/ci.nix
index 5dacffd..1e53108 100644
--- a/nix/ci.nix
+++ b/nix/ci.nix
@@ -5,13 +5,6 @@ let
 in
 
 dimension "Nixpkgs version" {
-    # avoid bitrotting the docker support (as opposed to podman)
-    "nixos-20_09" = {
-      nixpkgsSource = "nixos-20.09";
-      enableDoc = true;
-      dockerSupportsSystemd = true;
-      nixosHasPodmanDockerSocket = false;
-    };
     "nixos-22_05" = {
       nixpkgsSource = "nixos-22.05";
       enableDoc = true;
diff --git a/nix/overlay.nix b/nix/overlay.nix
index bbacd30..ecdb6af 100644
--- a/nix/overlay.nix
+++ b/nix/overlay.nix
@@ -49,6 +49,7 @@ in
         haskellPkgs.haskell-language-server
         super.docker-compose
         self.niv
+        self.nixpkgs-fmt
         self.releaser
       ];
     };
diff --git a/nix/sources.json b/nix/sources.json
index 6b3e284..037ef0c 100644
--- a/nix/sources.json
+++ b/nix/sources.json
@@ -11,18 +11,6 @@
         "url": "https://github.com/nmattia/niv/archive/fad2a6cbfb2e7cdebb7cb0ad2f5cc91e2c9bc06b.tar.gz",
         "url_template": "https://github.com/<owner>/<repo>/archive/<rev>.tar.gz"
     },
-    "nixos-20.09": {
-        "branch": "nixos-20.09",
-        "description": "Nix Packages collection",
-        "homepage": null,
-        "owner": "NixOS",
-        "repo": "nixpkgs",
-        "rev": "0cfe5377e8993052f9b0dd56d058f8008af45bd9",
-        "sha256": "0i3ybddi2mrlaz3di3svdpgy93zwmdglpywih4s9rd3wj865gzn1",
-        "type": "tarball",
-        "url": "https://github.com/NixOS/nixpkgs/archive/0cfe5377e8993052f9b0dd56d058f8008af45bd9.tar.gz",
-        "url_template": "https://github.com/<owner>/<repo>/archive/<rev>.tar.gz"
-    },
     "nixos-22.05": {
         "branch": "nixos-22.05",
         "description": "Nix Packages collection",
diff --git a/nix/upstreamable/default.nix b/nix/upstreamable/default.nix
index a06b42d..b82bc82 100644
--- a/nix/upstreamable/default.nix
+++ b/nix/upstreamable/default.nix
@@ -79,9 +79,7 @@ let
   /* Function to derivation of the docker compose yaml file
      NOTE: The output will change: https://github.com/hercules-ci/arion/issues/82
 
-    This function is particularly useful on CI. On Nixpkgs >= 20.09 this will
-    not store the image tarballs but executables to produce them reliably via
-    streamLayeredImage.
+     This function is particularly useful on CI.
    */
   build = args@{...}:
     let composition = eval args;
diff --git a/src/haskell/exe/Main.hs b/src/haskell/exe/Main.hs
index eeceda1..3fa7f19 100644
--- a/src/haskell/exe/Main.hs
+++ b/src/haskell/exe/Main.hs
@@ -60,7 +60,7 @@ parseOptions = do
           <> help "Use Nix expression EXPR to get the Nixpkgs attrset used for bootstrapping \
                    \and evaluating the configuration." )
     showTrace <- flag False True (long "show-trace"
-                    <> help "Causes Nix to print out a stack trace in case of Nix expression evaluation errors.")
+                    <> help "Causes Nix to print out a stack trace in case of Nix expression evaluation errors. Specify before command.")
     -- TODO --option support (https://github.com/pcapriotti/optparse-applicative/issues/284)
     userNixArgs <- many (T.pack <$> strOption (long "nix-arg" <> metavar "ARG" <> help "Pass an extra argument to nix. Example: --nix-arg --option --nix-arg substitute --nix-arg false"))
     prebuiltComposeFile <- optional $ strOption
diff --git a/src/haskell/testdata/Arion/NixSpec/arion-compose.json b/src/haskell/testdata/Arion/NixSpec/arion-compose.json
index 972db6f..5f644ac 100644
--- a/src/haskell/testdata/Arion/NixSpec/arion-compose.json
+++ b/src/haskell/testdata/Arion/NixSpec/arion-compose.json
@@ -1,4 +1,9 @@
 {
+    "networks": {
+        "default": {
+            "name": "unit-test-data"
+        }
+    },
     "services": {
         "webserver": {
             "command": [
@@ -37,7 +42,7 @@
             }
         ],
         "project": {
-            "name": null
+            "name": "unit-test-data"
         },
         "serviceInfo": {
             "webserver": {
diff --git a/src/haskell/testdata/Arion/NixSpec/arion-compose.nix b/src/haskell/testdata/Arion/NixSpec/arion-compose.nix
index dac420d..40c5907 100644
--- a/src/haskell/testdata/Arion/NixSpec/arion-compose.nix
+++ b/src/haskell/testdata/Arion/NixSpec/arion-compose.nix
@@ -1,4 +1,5 @@
 {
+  project.name = "unit-test-data";
   services.webserver = { pkgs, ... }: {
     nixos.useSystemd = true;
     nixos.configuration.boot.tmpOnTmpfs = true;
diff --git a/src/nix/lib.nix b/src/nix/lib.nix
new file mode 100644
index 0000000..1e3ede6
--- /dev/null
+++ b/src/nix/lib.nix
@@ -0,0 +1,16 @@
+{ lib }:
+let
+
+  link = url: text:
+    ''link:${url}[${text}]'';
+
+  dockerComposeRef = fragment:
+    ''See ${link "https://docs.docker.com/compose/compose-file/#${fragment}" "Docker Compose#${fragment}"}'';
+
+in
+{
+  inherit
+    dockerComposeRef
+    link
+    ;
+}
diff --git a/src/nix/modules.nix b/src/nix/modules.nix
index ecc73c1..8c3251a 100644
--- a/src/nix/modules.nix
+++ b/src/nix/modules.nix
@@ -2,6 +2,7 @@
     ./modules/composition/docker-compose.nix
     ./modules/composition/host-environment.nix
     ./modules/composition/images.nix
+    ./modules/composition/networks.nix
     ./modules/composition/service-info.nix
     ./modules/composition/composition.nix
 ]
\ No newline at end of file
diff --git a/src/nix/modules/composition/composition.nix b/src/nix/modules/composition/composition.nix
index 37d2854..8b53882 100644
--- a/src/nix/modules/composition/composition.nix
+++ b/src/nix/modules/composition/composition.nix
@@ -16,9 +16,10 @@ in
         Name of the project.
 
         See ${link "https://docs.docker.com/compose/reference/envvars/#compose_project_name" "COMPOSE_PROJECT_NAME"}
+
+        This is not optional, because getting the project name from a directory name tends to produce different results for different repo checkout location names.
       '';
-      type = types.nullOr types.str;
-      default = null;
+      type = types.str;
     };
   };
   config = {
diff --git a/src/nix/modules/composition/networks.nix b/src/nix/modules/composition/networks.nix
new file mode 100644
index 0000000..15435d9
--- /dev/null
+++ b/src/nix/modules/composition/networks.nix
@@ -0,0 +1,53 @@
+{ config, lib, ... }:
+
+let
+  inherit (lib)
+    mkOption
+    optionalAttrs
+    types
+    ;
+  inherit (import ../../lib.nix { inherit lib; })
+    dockerComposeRef
+    ;
+in
+{
+  options = {
+    networks = mkOption {
+      type = types.lazyAttrsOf (types.submoduleWith {
+        modules = [
+          ../networks/network.nix
+        ];
+      });
+      description = ''
+        ${dockerComposeRef "networks-top-level-element"}
+      '';
+    };
+    enableDefaultNetwork = mkOption {
+      type = types.bool;
+      description = ''
+        Whether to define the default network:
+
+        ```nix
+        networks.default = {
+          name = config.project.name;
+        };
+        ```
+      '';
+      default = true;
+    };
+  };
+
+
+  config = {
+
+    networks = optionalAttrs config.enableDefaultNetwork {
+      default = {
+        name = config.project.name;
+      };
+    };
+
+    docker-compose.raw.networks =
+      lib.mapAttrs (k: v: v.out) config.networks;
+
+  };
+}
diff --git a/src/nix/modules/networks/network.nix b/src/nix/modules/networks/network.nix
new file mode 100644
index 0000000..16083b8
--- /dev/null
+++ b/src/nix/modules/networks/network.nix
@@ -0,0 +1,131 @@
+{ config, lib, options, ... }:
+
+let
+  inherit (lib)
+    mkOption
+    optionalAttrs
+    types
+    ;
+  inherit (import ../../lib.nix { inherit lib; })
+    dockerComposeRef
+    ;
+in
+{
+  options = {
+    driver = mkOption {
+      description = ''
+        `"none"`, `"host"`, or a platform-specific value.
+        ${dockerComposeRef "driver"}
+      '';
+      type = types.str;
+    };
+
+    driver_opts = mkOption {
+      description = ''
+        ${dockerComposeRef "driver_opts"}
+      '';
+      type = types.lazyAttrsOf types.raw or types.unspecified;
+    };
+
+    attachable = mkOption {
+      description = ''
+        ${dockerComposeRef "attachable"}
+      '';
+      type = types.bool;
+      example = true;
+    };
+
+    enable_ipv6 = mkOption {
+      description = ''
+        Whether we've entered the 21st century yet.
+        
+        ${dockerComposeRef "enable_ipv6"}
+      '';
+      type = types.bool;
+    };
+
+    ipam = mkOption {
+      # TODO model sub-options
+      description = ''
+        Manage IP addresses.
+
+        ${dockerComposeRef "ipam"}
+      '';
+      type = types.raw or types.unspecified;
+    };
+
+    internal = mkOption {
+      description = ''
+        Achieves "external isolation".
+
+        ${dockerComposeRef "internal"}
+      '';
+      defaultText = false;
+      type = types.bool;
+    };
+
+    labels = mkOption {
+      description = ''
+        Metadata.
+
+        ${dockerComposeRef "labels"}
+      '';
+      # no list support, because less expressive wrt overriding
+      type = types.attrsOf types.str;
+    };
+
+    external = mkOption {
+      description = ''
+        When `true`, don't create or destroy the network, but assume that it
+        exists.
+
+        ${dockerComposeRef "external"}
+      '';
+      type = types.bool;
+    };
+
+    name = mkOption {
+      description = ''
+        Set a custom name for the network.
+
+        It shares a namespace with other projects' networks. `name` is used as-is.
+
+        Note the `default` network's default `name` is set to `project.name` by Arion.
+
+        ${dockerComposeRef "name"}
+      '';
+      type = types.str;
+    };
+
+    out = mkOption {
+      internal = true;
+      description = ''
+        This network's contribution to the docker compose yaml file
+        under the `networks.''${name}` key.
+      '';
+      type = lib.types.attrsOf lib.types.raw or lib.types.unspecified;
+    };
+  };
+
+  config = {
+    out =
+      lib.mapAttrs
+        (k: opt: opt.value)
+        (lib.filterAttrs
+          (k: opt: builtins.trace k builtins.trace opt builtins.trace "" opt.isDefined)
+          {
+            inherit (options)
+              driver
+              driver_opts
+              attachable
+              enable_ipv6
+              ipam
+              internal
+              labels
+              external
+              name
+              ;
+          }
+        );
+  };
+}
diff --git a/src/nix/modules/service/docker-compose-service.nix b/src/nix/modules/service/docker-compose-service.nix
index 6b64e84..350c215 100644
--- a/src/nix/modules/service/docker-compose-service.nix
+++ b/src/nix/modules/service/docker-compose-service.nix
@@ -10,10 +10,11 @@ let
   inherit (lib) mkOption types;
   inherit (types) listOf nullOr attrsOf str either int bool submodule enum;
 
-  link = url: text:
-    ''link:${url}[${text}]'';
-  dockerComposeRef = fragment:
-    ''See ${link "https://docs.docker.com/compose/compose-file/#${fragment}" "Docker Compose#${fragment}"}'';
+  inherit (import ../../lib.nix { inherit lib; })
+    link
+    dockerComposeRef
+    ;
+
   dockerComposeKitchenSink = ''
     Analogous to the `docker run` counterpart.