diff --git a/container/docker/handler.go b/container/docker/handler.go
index 065b19da..8453d031 100644
--- a/container/docker/handler.go
+++ b/container/docker/handler.go
@@ -157,12 +157,12 @@ func libcontainerConfigToContainerSpec(config *libcontainer.Config, mi *info.Mac
 	if config.Cgroups.CpuShares != 0 {
 		spec.Cpu.Limit = uint64(config.Cgroups.CpuShares)
 	}
-	n := (mi.NumCores + 63) / 64
-	spec.Cpu.Mask.Data = make([]uint64, n)
-	for i := 0; i < n; i++ {
-		spec.Cpu.Mask.Data[i] = math.MaxUint64
+	if config.Cgroups.CpusetCpus == "" {
+		// All cores are active.
+		spec.Cpu.Mask = fmt.Sprintf("0-%d", mi.NumCores-1)
+	} else {
+		spec.Cpu.Mask = config.Cgroups.CpusetCpus
 	}
-	// TODO(vmarmol): Get CPUs from config.Cgroups.CpusetCpus
 	return spec
 }
 
diff --git a/container/raw/handler.go b/container/raw/handler.go
index 4345f0bf..9988bea5 100644
--- a/container/raw/handler.go
+++ b/container/raw/handler.go
@@ -15,9 +15,9 @@
 package raw
 
 import (
+	"fmt"
 	"io/ioutil"
 	"log"
-	"math"
 	"path/filepath"
 	"strconv"
 	"strings"
@@ -102,11 +102,7 @@ func (self *rawContainerHandler) GetSpec() (*info.ContainerSpec, error) {
 			spec.Cpu.Limit = readInt64(cpuRoot, "cpu.shares")
 
 			// TODO(vmarmol): Get CPUs from config.Cgroups.CpusetCpus
-			n := (mi.NumCores + 63) / 64
-			spec.Cpu.Mask.Data = make([]uint64, n)
-			for i := 0; i < n; i++ {
-				spec.Cpu.Mask.Data[i] = math.MaxUint64
-			}
+			spec.Cpu.Mask = fmt.Sprintf("0-%d", mi.NumCores-1)
 		}
 	}
 
diff --git a/info/container.go b/info/container.go
index 48252a9c..d86bdf96 100644
--- a/info/container.go
+++ b/info/container.go
@@ -21,14 +21,10 @@ import (
 	"time"
 )
 
-type CpuSpecMask struct {
-	Data []uint64 `json:"data,omitempty"`
-}
-
 type CpuSpec struct {
-	Limit    uint64      `json:"limit"`
-	MaxLimit uint64      `json:"max_limit"`
-	Mask     CpuSpecMask `json:"mask,omitempty"`
+	Limit    uint64 `json:"limit"`
+	MaxLimit uint64 `json:"max_limit"`
+	Mask     string `json:"mask,omitempty"`
 }
 
 type MemorySpec struct {
diff --git a/pages/containers.go b/pages/containers.go
index be7c2e26..6d7b40c9 100644
--- a/pages/containers.go
+++ b/pages/containers.go
@@ -88,18 +88,12 @@ func containerLink(container info.ContainerReference, basenameOnly bool, cssClas
 	return template.HTML(fmt.Sprintf("<a class=\"%s\" href=\"%s%s\">%s</a>", cssClasses, ContainersPage[:len(ContainersPage)-1], containerName, displayName))
 }
 
-func printMask(mask *info.CpuSpecMask, numCores int) interface{} {
-	// TODO(vmarmol): Detect this correctly.
-	// TODO(vmarmol): Support more than 64 cores.
-	rawMask := uint64(0)
-	if len(mask.Data) > 0 {
-		rawMask = mask.Data[0]
-	}
+func printMask(mask string, numCores int) interface{} {
 	masks := make([]string, numCores)
-	for i := uint(0); i < uint(numCores); i++ {
+	activeCores := getActiveCores(mask)
+	for i := 0; i < numCores; i++ {
 		coreClass := "inactive-cpu"
-		// by default, all cores are active
-		if ((0x1<<i)&rawMask) != 0 || len(mask.Data) == 0 {
+		if activeCores[i] {
 			coreClass = "active-cpu"
 		}
 		masks[i] = fmt.Sprintf("<span class=\"%s\">%d</span>", coreClass, i)
@@ -107,6 +101,34 @@ func printMask(mask *info.CpuSpecMask, numCores int) interface{} {
 	return template.HTML(strings.Join(masks, "&nbsp;"))
 }
 
+func getActiveCores(mask string) map[int]bool {
+	activeCores := make(map[int]bool)
+	for _, corebits := range strings.Split(mask, ",") {
+		cores := strings.Split(corebits, "-")
+		if len(cores) == 1 {
+			index, err := strconv.Atoi(cores[0])
+			if err != nil {
+				// Ignore malformed strings.
+				continue
+			}
+			activeCores[index] = true
+		} else if len(cores) == 2 {
+			start, err := strconv.Atoi(cores[0])
+			if err != nil {
+				continue
+			}
+			end, err := strconv.Atoi(cores[1])
+			if err != nil {
+				continue
+			}
+			for i := start; i <= end; i++ {
+				activeCores[i] = true
+			}
+		}
+	}
+	return activeCores
+}
+
 func printCores(millicores *uint64) string {
 	// TODO(vmarmol): Detect this correctly
 	if *millicores > 1024*1000 {