During test case execution, we came across all test case failures due to invalid IP.s generation for s390x system. On checking git issues , we came across the link https://github.com/coreos/flannel/issues/526, which logs similar issue for bigEndiane during IP.s generation.

      On analysis we came across littleEndian check, where code is manipulating IP.s for bigEndianess, which is unnecessary because changes are required only at bit-level not for IP. We are removing the littleEndian check where the IP address are involved.
      For example subnetSize := ip.IP4(1 << (32 - cfg.SubnetLen)), assume cfg.SubnetLen=24 (by default) then, value of subnetSize should be same regardless of endianess, which is achieved after endiness check is removed.

pkg/ip/ipnet.go

@@ -24,17 +24,10 @@ import (
 type IP4 uint32
 
 func FromBytes(ip []byte) IP4 {
-	if NativelyLittle() {
-		return IP4(uint32(ip[3]) |
-			(uint32(ip[2]) << 8) |
-			(uint32(ip[1]) << 16) |
-			(uint32(ip[0]) << 24))
-	} else {
-		return IP4(uint32(ip[0]) |
-			(uint32(ip[1]) << 8) |
-			(uint32(ip[2]) << 16) |
-			(uint32(ip[3]) << 24))
-	}
+	return IP4(uint32(ip[3]) |
+		(uint32(ip[2]) << 8) |
+		(uint32(ip[1]) << 16) |
+		(uint32(ip[0]) << 24))
 }
 
 func FromIP(ip net.IP) IP4 {
@@ -58,11 +51,7 @@ func MustParseIP4(s string) IP4 {
 }
 
 func (ip IP4) Octets() (a, b, c, d byte) {
-	if NativelyLittle() {
 		a, b, c, d = byte(ip>>24), byte(ip>>16), byte(ip>>8), byte(ip)
-	} else {
-		a, b, c, d = byte(ip), byte(ip>>8), byte(ip>>16), byte(ip>>24)
-	}
 	return
 }
 
@@ -71,12 +60,8 @@ func (ip IP4) ToIP() net.IP {
 }
 
 func (ip IP4) NetworkOrder() uint32 {
-	if NativelyLittle() {
-		a, b, c, d := byte(ip>>24), byte(ip>>16), byte(ip>>8), byte(ip)
-		return uint32(a) | (uint32(b) << 8) | (uint32(c) << 16) | (uint32(d) << 24)
-	} else {
-		return uint32(ip)
-	}
+	a, b, c, d := byte(ip>>24), byte(ip>>16), byte(ip>>8), byte(ip)
+	return uint32(a) | (uint32(b) << 8) | (uint32(c) << 16) | (uint32(d) << 24)
 }
 
 func (ip IP4) String() string {