diff --git a/peeringdb_server/management/commands/pdb_api_test.py b/peeringdb_server/management/commands/pdb_api_test.py
index 9a70c56a..a4a7d8a2 100644
--- a/peeringdb_server/management/commands/pdb_api_test.py
+++ b/peeringdb_server/management/commands/pdb_api_test.py
@@ -1329,7 +1329,7 @@ class TestJSON(unittest.TestCase):
         #}, test_success=False)
 
         self.assert_update(self.db_org_admin, "ixpfx", SHARED["ixpfx_id"],
-                           {"prefix": "206.126.236.0/24"}, test_failures={
+                           {"prefix": "206.127.236.0/26"}, test_failures={
                                "invalid": {
                                    "prefix": "NEEDS TO BE VALID PREFIX"
                                },
diff --git a/peeringdb_server/models.py b/peeringdb_server/models.py
index 65f480f6..208c47d0 100644
--- a/peeringdb_server/models.py
+++ b/peeringdb_server/models.py
@@ -37,7 +37,8 @@ from passlib.hash import sha256_crypt
 
 from peeringdb_server.inet import RdapLookup, RdapNotFoundError
 from peeringdb_server.validators import (
-    validate_address_space, validate_info_prefixes4, validate_info_prefixes6)
+    validate_address_space, validate_info_prefixes4, validate_info_prefixes6,
+    validate_prefix_overlap)
 
 SPONSORSHIP_LEVELS = ((1, _("Silver")), (2, _("Gold")), (3, _("Platinum")),
                       (4, _("Diamond")))
@@ -1775,12 +1776,14 @@ class IXLanPrefix(pdb_models.IXLanPrefixBase):
         except ValueError as inst:
             return False
 
+
     def clean(self):
         """
         Custom model validation
         """
         # validate the specified prefix address
         validate_address_space(self.prefix)
+        validate_prefix_overlap(self.prefix)
         return super(IXLanPrefix, self).clean()
 
 
diff --git a/peeringdb_server/serializers.py b/peeringdb_server/serializers.py
index 0796652e..81a530fe 100644
--- a/peeringdb_server/serializers.py
+++ b/peeringdb_server/serializers.py
@@ -27,7 +27,8 @@ from peeringdb_server.models import (
     InternetExchangeFacility, IXLan, IXLanPrefix, Facility, Network,
     NetworkContact, NetworkFacility, NetworkIXLan, Organization)
 from peeringdb_server.validators import (
-    validate_address_space, validate_info_prefixes4, validate_info_prefixes6)
+    validate_address_space, validate_info_prefixes4, validate_info_prefixes6,
+    validate_prefix_overlap)
 
 from django.utils.translation import ugettext_lazy as _
 
@@ -1461,7 +1462,9 @@ class IXLanPrefixSerializer(ModelSerializer):
 
     prefix = IPPrefixField(validators=[
         validators.UniqueValidator(
-            queryset=IXLanPrefix.objects.all()), validate_address_space
+            queryset=IXLanPrefix.objects.all()),
+            validate_address_space,
+            validate_prefix_overlap,
     ])
 
     class Meta:
@@ -1614,7 +1617,8 @@ class InternetExchangeSerializer(ModelSerializer):
     prefix = IPPrefixField(validators=[
         validators.UniqueValidator(
             queryset=IXLanPrefix.objects.filter(status__in=["ok", "pending"])),
-        validate_address_space
+        validate_address_space,
+        validate_prefix_overlap,
     ], required=False, write_only=True)
 
     validators = [
diff --git a/peeringdb_server/static/site.css b/peeringdb_server/static/site.css
index fdd8516f..d6329034 100644
--- a/peeringdb_server/static/site.css
+++ b/peeringdb_server/static/site.css
@@ -842,6 +842,12 @@ div.editable.mode-edit div.view_field {
   background-color: #9e1e1e;
 }
 
+.input-note-long .input-note {
+  white-space:normal;
+  top:auto;
+  bottom:25px;
+}
+
 div.view > div.editable.popin.error {
   margin-top: 20px;
   max-width: 600px;
diff --git a/peeringdb_server/templates/site/view_exchange_bottom.html b/peeringdb_server/templates/site/view_exchange_bottom.html
index a4721c4c..5eb8ce35 100644
--- a/peeringdb_server/templates/site/view_exchange_bottom.html
+++ b/peeringdb_server/templates/site/view_exchange_bottom.html
@@ -145,7 +145,7 @@
                  data-edit-name="protocol"
                  data-edit-data="enum/protocols">
             </div>
-            <div class="col-xs-5 col-sm-5 col-md-5"
+            <div class="col-xs-5 col-sm-5 col-md-5 input-note-long"
                  data-edit-type="string"
                  data-edit-name="prefix"
                  data-edit-placeholder="{% trans "Prefix" %}"
diff --git a/peeringdb_server/validators.py b/peeringdb_server/validators.py
index 6a04848b..e71ad150 100644
--- a/peeringdb_server/validators.py
+++ b/peeringdb_server/validators.py
@@ -9,11 +9,33 @@ from django.core.exceptions import ValidationError
 from django.utils.translation import ugettext_lazy as _
 
 from peeringdb_server.inet import network_is_pdb_valid
+import peeringdb_server.models
+
+def validate_prefix(prefix):
+    """
+    validate ip prefix
+
+    Arguments:
+        - prefix: ipaddress.IPv4Network or an ipaddress.IPv6Network
+
+    Raises:
+        - ValidationError on failed validation
+
+    Returns:
+        - ipaddress.ip_network instance
+    """
+
+    if isinstance(prefix, unicode):
+        try:
+            prefix = ipaddress.ip_network(prefix)
+        except ValueError as exc:
+            raise ValidationError(_("Invalid prefix: {}").format(prefix))
+    return prefix
 
 
 def validate_address_space(prefix):
     """
-    validate an ipaddress according to peeringdb specs
+    validate an ip prefix according to peeringdb specs
 
     Arguments:
         - prefix: ipaddress.IPv4Network or an ipaddress.IPv6Network
@@ -22,11 +44,7 @@ def validate_address_space(prefix):
         - ValidationError on failed validation
     """
 
-    if isinstance(prefix, unicode):
-        try:
-            prefix = ipaddress.ip_network(prefix)
-        except ValueError as exc:
-            raise ValidationError(_("Invalid prefix: {}").format(prefix))
+    prefix = validate_prefix(prefix)
 
     if not network_is_pdb_valid(prefix):
         raise ValidationError(_("Address space invalid: {}").format(prefix))
@@ -58,3 +76,29 @@ def validate_info_prefixes6(value):
 
     if value < 0:
         raise ValidationError(_("Negative value not allowed"))
+
+
+def validate_prefix_overlap(prefix):
+    """
+    validate that a prefix does not overlap with another prefix
+    on an already existing ixlan
+
+    Arguments:
+        - prefix: ipaddress.IPv4Network or an ipaddress.IPv6Network
+
+    Raises:
+        - ValidationError on failed validation
+    """
+
+
+    prefix = validate_prefix(prefix)
+    qs = peeringdb_server.models.IXLanPrefix.objects.filter(protocol="IPv{}".format(prefix.version),
+                                                            status="ok")
+    qs = qs.exclude(prefix=prefix)
+    for ixpfx in qs:
+        if ixpfx.prefix.overlaps(prefix):
+            raise ValidationError(_("Prefix overlaps with {}'s prefix: {}".format(
+                ixpfx.ixlan.ix.name, ixpfx.prefix
+            )))
+
+
diff --git a/tests/test_validators.py b/tests/test_validators.py
index 70bd27e5..53f01849 100644
--- a/tests/test_validators.py
+++ b/tests/test_validators.py
@@ -7,7 +7,14 @@ from django.test import override_settings
 from django.core.exceptions import ValidationError
 
 from peeringdb_server.validators import (
-    validate_address_space, validate_info_prefixes4, validate_info_prefixes6)
+    validate_address_space, validate_info_prefixes4, validate_info_prefixes6,
+    validate_prefix_overlap)
+
+from peeringdb_server.models import (
+    Organization,
+    InternetExchange,
+    IXLan,
+    IXLanPrefix)
 
 pytestmark = pytest.mark.django_db
 
@@ -118,3 +125,16 @@ def test_validate_prefixlen():
         validate_address_space(u"2403:c240::/32")
     with pytest.raises(ValidationError):
         validate_address_space(u"2001:504:0:2::/64")
+
+@pytest.mark.djangodb
+def test_validate_prefix_overlap():
+    org = Organization.objects.create(name="Test org",status="ok")
+    ix = InternetExchange.objects.create(name="Text exchange",status="ok",org=org)
+    ixlan = IXLan.objects.create(ix=ix, status="ok")
+
+    pfx1 = IXLanPrefix.objects.create(ixlan=ixlan, protocol="IPv4",
+                                      prefix=ipaddress.ip_network(u"198.32.125.0/24"),
+                                      status="ok")
+
+    with pytest.raises(ValidationError) as exc:
+        validate_prefix_overlap(u"198.32.124.0/23")