diff --git a/moto/ec2/models.py b/moto/ec2/models.py
index be39bab28..2611c2f1a 100644
--- a/moto/ec2/models.py
+++ b/moto/ec2/models.py
@@ -775,7 +775,14 @@ class Instance(TaggedEC2Resource, BotoInstance):
                 if "SubnetId" in nic:
                     subnet = self.ec2_backend.get_subnet(nic["SubnetId"])
                 else:
-                    subnet = None
+                    # Get default Subnet
+                    subnet = [
+                        subnet
+                        for subnet in self.ec2_backend.get_all_subnets(
+                            filters={"availabilityZone": self._placement.zone}
+                        )
+                        if subnet.default_for_az
+                    ][0]
 
                 group_id = nic.get("SecurityGroupId")
                 group_ids = [group_id] if group_id else []
diff --git a/tests/test_ec2/test_instances.py b/tests/test_ec2/test_instances.py
index 85ba0fe01..fe1631223 100644
--- a/tests/test_ec2/test_instances.py
+++ b/tests/test_ec2/test_instances.py
@@ -71,7 +71,7 @@ def test_instance_launch_and_terminate():
     instance.id.should.equal(instance.id)
     instance.state.should.equal("running")
     instance.launch_time.should.equal("2014-01-01T05:00:00.000Z")
-    instance.vpc_id.should.equal(None)
+    instance.vpc_id.shouldnt.equal(None)
     instance.placement.should.equal("us-east-1a")
 
     root_device_name = instance.root_device_name
diff --git a/tests/test_ec2/test_subnets.py b/tests/test_ec2/test_subnets.py
index 7bb57aab4..a16693d5f 100644
--- a/tests/test_ec2/test_subnets.py
+++ b/tests/test_ec2/test_subnets.py
@@ -599,3 +599,50 @@ def validate_subnet_details_after_creating_eni(
     for eni in enis_created:
         client.delete_network_interface(NetworkInterfaceId=eni["NetworkInterfaceId"])
     client.delete_subnet(SubnetId=subnet["SubnetId"])
+
+
+@mock_ec2
+def test_run_instances_should_attach_to_default_subnet():
+    ec2 = boto3.resource("ec2", region_name="us-west-1")
+    client = boto3.client("ec2", region_name="us-west-1")
+    ec2.create_security_group(GroupName="sg01", Description="Test security group sg01")
+    # run_instances
+    instances = client.run_instances(
+        MinCount=1,
+        MaxCount=1,
+        SecurityGroups=["sg01"],
+        TagSpecifications=[
+            {
+                "ResourceType": "instance",
+                "Tags": [{"Key": "Name", "Value": "test-01"},],
+            }
+        ],
+    )
+    default_subnet_id = client.describe_subnets()["Subnets"][0]["SubnetId"]
+    instances["Instances"][0]["NetworkInterfaces"][0]["SubnetId"].should.equal(
+        default_subnet_id
+    )
+
+
+@mock_ec2
+def test_describe_subnets_where_network_interface_has_no_subnets_attached():
+    # https://github.com/spulec/moto/issues/2877
+    # create security groups
+    ec2 = boto3.resource("ec2", region_name="us-west-1")
+    client = boto3.client("ec2", region_name="us-west-1")
+    ec2.create_security_group(GroupName="sg01", Description="Test security group sg01")
+    # run_instances
+    client.run_instances(
+        MinCount=1,
+        MaxCount=1,
+        SecurityGroups=["sg01"],
+        TagSpecifications=[
+            {
+                "ResourceType": "instance",
+                "Tags": [{"Key": "Name", "Value": "test-01"},],
+            }
+        ],
+    )
+    # describe_subnets
+    subnets = client.describe_subnets()["Subnets"]
+    subnets[0]["AvailableIpAddressCount"].should.equal(4090)