diff --git a/format/plain/writer.go b/format/plain/writer.go
index 49f417e..deeb5a1 100644
--- a/format/plain/writer.go
+++ b/format/plain/writer.go
@@ -42,13 +42,6 @@ func NewWriter(meta *model.Meta) (*Writer, error) {
 		meta: meta,
 	}
 
-	ret.buffer = bytes.NewBuffer([]byte{})
-	ret.iw = ret.buffer
-
-	if err := ret.Header(); err != nil {
-		return nil, err
-	}
-
 	return ret, nil
 }
 
@@ -76,7 +69,12 @@ func (w *Writer) SetOption(option interface{}) error {
 // Insert adds the given IP information into the writer.
 func (w *Writer) Insert(info *model.IPInfo) error {
 	if w.iw == nil {
-		return errors.ErrNilWriter
+		w.buffer = bytes.NewBuffer([]byte{})
+		w.iw = w.buffer
+
+		if err := w.Header(); err != nil {
+			return err
+		}
 	}
 
 	for _, ipNet := range info.IPNet.IPNets() {
diff --git a/format/zxinc/reader.go b/format/zxinc/reader.go
index 7af7c82..55e718d 100644
--- a/format/zxinc/reader.go
+++ b/format/zxinc/reader.go
@@ -45,7 +45,7 @@ func NewReader(file string) (*Reader, error) {
 	meta := &model.Meta{
 		MetaVersion: model.MetaVersion,
 		Format:      DBFormat,
-		IPVersion:   model.IPv4,
+		IPVersion:   model.IPv6,
 		Fields:      FullFields,
 	}
 	meta.AddCommonFieldAlias(CommonFieldsAlias)
diff --git a/ipnet/ip.go b/ipnet/ip.go
index 860eb9c..23fe064 100644
--- a/ipnet/ip.go
+++ b/ipnet/ip.go
@@ -78,32 +78,46 @@ func Uint64ToIP2(high, low uint64) net.IP {
 	}
 }
 
-// adjustIP modifies the IP by the given delta, which can be positive or negative.
-func adjustIP(ip net.IP, delta int) net.IP {
-	ipVals := make([]byte, len(ip))
-	copy(ipVals, ip)
-	i := len(ipVals) - 1
-	for i >= 0 {
-		sum := int(ipVals[i]) + delta
-		if 0 <= sum && sum <= 255 {
-			ipVals[i] = byte(sum)
+// PrevIP returns the IP immediately before the given IP.
+func PrevIP(ip net.IP) net.IP {
+	res := make(net.IP, len(ip))
+	copy(res, ip)
+
+	// Ensure it's a pure IPv4 or IPv6
+	if ip.To4() != nil {
+		ip = ip.To4()
+		res = res[len(res)-4:]
+	}
+
+	for i := len(ip) - 1; i >= 0; i-- {
+		if res[i] > 0 {
+			res[i]--
 			break
 		}
-		// Adjust the next byte and continue
-		delta, ipVals[i] = sum/256, byte(sum%256)
-		i--
+		res[i] = 0xff
 	}
-	return net.IP(ipVals)
-}
-
-// PrevIP returns the IP immediately before the given IP.
-func PrevIP(ip net.IP) net.IP {
-	return adjustIP(ip, -1)
+	return res
 }
 
 // NextIP returns the IP immediately after the given IP.
 func NextIP(ip net.IP) net.IP {
-	return adjustIP(ip, 1)
+	res := make(net.IP, len(ip))
+	copy(res, ip)
+
+	// Ensure it's a pure IPv4 or IPv6
+	if ip.To4() != nil {
+		ip = ip.To4()
+		res = res[len(res)-4:]
+	}
+
+	for i := len(ip) - 1; i >= 0; i-- {
+		if res[i] < 0xff {
+			res[i]++
+			break
+		}
+		res[i] = 0
+	}
+	return res
 }
 
 // IPLess compares two IPs and returns true if the first IP is less than the second.
diff --git a/ipnet/ip_test.go b/ipnet/ip_test.go
index 62ffb70..18d1107 100644
--- a/ipnet/ip_test.go
+++ b/ipnet/ip_test.go
@@ -18,6 +18,7 @@ package ipnet
 
 import (
 	"math/rand"
+	"net"
 	"testing"
 
 	"github.com/stretchr/testify/assert"
@@ -43,3 +44,43 @@ func TestIPv4StrToUint32(t *testing.T) {
 	i = IPv4StrToUint32("fake")
 	ast.Equal(uint32(0), i)
 }
+
+func TestPrevIP(t *testing.T) {
+	tests := []struct {
+		input    net.IP
+		expected net.IP
+	}{
+		{net.ParseIP("192.168.1.1"), net.ParseIP("192.168.1.0")},
+		{net.ParseIP("192.168.1.0"), net.ParseIP("192.168.0.255")},
+		{net.ParseIP("0.0.0.0"), net.ParseIP("255.255.255.255")},
+		{net.ParseIP("::1"), net.ParseIP("::")},
+		{net.ParseIP("::"), net.ParseIP("ffff:ffff:ffff:ffff:ffff:ffff:ffff:ffff")},
+	}
+
+	for _, test := range tests {
+		output := PrevIP(test.input)
+		if !output.Equal(test.expected) {
+			t.Errorf("For input %v, expected %v, but got %v", test.input, test.expected, output)
+		}
+	}
+}
+
+func TestNextIP(t *testing.T) {
+	tests := []struct {
+		input    net.IP
+		expected net.IP
+	}{
+		{net.ParseIP("192.168.1.0"), net.ParseIP("192.168.1.1")},
+		{net.ParseIP("192.168.0.255"), net.ParseIP("192.168.1.0")},
+		{net.ParseIP("255.255.255.255"), net.ParseIP("0.0.0.0")},
+		{net.ParseIP("::"), net.ParseIP("::1")},
+		{net.ParseIP("ffff:ffff:ffff:ffff:ffff:ffff:ffff:ffff"), net.ParseIP("::")},
+	}
+
+	for _, test := range tests {
+		output := NextIP(test.input)
+		if !output.Equal(test.expected) {
+			t.Errorf("For input %v, expected %v, but got %v", test.input, test.expected, output)
+		}
+	}
+}