diff --git a/.gitignore b/.gitignore
index 31c0992..a1ebf22 100644
--- a/.gitignore
+++ b/.gitignore
@@ -1,5 +1,7 @@
 .idea
 *.mmdb
+# Ignore all mmdb files in data-snapshots directory, used for unit-testing
+!data-snapshots/*.mmdb
 **/venv/*
 **/__pycache__/*
 # zannotate binary
diff --git a/README.md b/README.md
index 745f477..86e8835 100644
--- a/README.md
+++ b/README.md
@@ -33,3 +33,25 @@ Check that it was installed correctly with:
 ```shell
 zannotate --help
 ```
+
+# Acquiring Datasets
+
+> [!NOTE]
+> URLs and instructions may change over time. These are up-to-date as of September 2025.
+
+Below are instructions for getting datasets from the below providers.
+
+### GeoLite2-ASN
+1. [Sign-up form](https://www.maxmind.com/en/geolite2/signup) for MaxMind GeoLite Access
+2. Login to your account
+3. Go to the "GeoIP / GeoLite" > "Download files" section where you should see a list of available databases
+4. Download the `.mmdb` files for GeoLite ASN
+5. Unzip the downloaded file and test with:
+
+```shell
+echo "1.1.1.1" | zannotate --geoasn --geoasn-database=/path-to-downloaded-file/GeoLite2-ASN_20250923/GeoLite2-ASN.mmdb
+```
+
+```shell
+{"ip":"1.1.1.1","geoasn":{"asn":13335,"org":"CLOUDFLARENET"}}
+```
diff --git a/data-snapshots/geolite2_asn.mmdb b/data-snapshots/geolite2_asn.mmdb
new file mode 100644
index 0000000..62987b7
Binary files /dev/null and b/data-snapshots/geolite2_asn.mmdb differ
diff --git a/geoipasn_test.go b/geoipasn_test.go
new file mode 100644
index 0000000..2665708
--- /dev/null
+++ b/geoipasn_test.go
@@ -0,0 +1,77 @@
+/*
+ * ZAnnotate Copyright 2025 Regents of the University of Michigan
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License"); you may not
+ * use this file except in compliance with the License. You may obtain a copy
+ * of the License at http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or
+ * implied. See the License for the specific language governing
+ * permissions and limitations under the License.
+ */
+
+package zannotate
+
+import (
+	"net"
+	"reflect"
+	"testing"
+)
+
+func TestGeoIPASNAnnotator(t *testing.T) {
+	tests := []struct {
+		testName       string
+		ipAddr         net.IP
+		expectedResult *GeoIPASNOutput
+	}{
+		{
+			testName: "Positive Test Case, IPv4",
+			ipAddr:   net.ParseIP("1.1.1.1"),
+			expectedResult: &GeoIPASNOutput{
+				ASN:    13335,
+				ASNOrg: "CLOUDFLARENET",
+			},
+		}, {
+			testName: "Positive Test Case, IPv6",
+			ipAddr:   net.ParseIP("2606:4700:4700::1111"),
+			expectedResult: &GeoIPASNOutput{
+				ASN:    13335,
+				ASNOrg: "CLOUDFLARENET",
+			},
+		}, {
+			testName:       "Negative Test Case, Invalid IP",
+			ipAddr:         net.ParseIP("999.999.999.999"),
+			expectedResult: &GeoIPASNOutput{},
+		}, {
+			testName:       "Negative Test Case, Private IP",
+			ipAddr:         net.ParseIP("127.0.0.1"),
+			expectedResult: &GeoIPASNOutput{},
+		},
+	}
+	factory := &GeoIPASNAnnotatorFactory{
+		Path: "./data-snapshots/geolite2_asn.mmdb",
+		Mode: "mmap",
+	}
+	err := factory.Initialize(nil)
+	if err != nil {
+		t.Fatalf("Failed to initialize factory: %v", err)
+	}
+	for _, tt := range tests {
+		t.Run(tt.testName, func(t *testing.T) {
+			annotator := factory.MakeAnnotator(0).(*GeoIPASNAnnotator)
+			err = annotator.Initialize()
+			if err != nil {
+				t.Fatalf("Failed to initialize annotator: %v", err)
+			}
+			result := annotator.Annotate(tt.ipAddr)
+			if tt.expectedResult == nil && result == nil {
+				return // pass
+			}
+			if !reflect.DeepEqual(result, tt.expectedResult) {
+				t.Errorf("Annotating IP %s gave = %v; expected: %v", tt.ipAddr, result, tt.expectedResult)
+			}
+		})
+	}
+}